Cloning - biology.

Cloning - biology. I INTRODUCTION Cloning, process of creating an exact copy of a single gene, cell, or organism. The copies produced through cloning have identical genetic makeup and are known as clones. Many organisms in nature reproduce by cloning. Scientists use cloning techniques in the laboratory to create copies of cells or organisms with valuable traits. Their work aims to find practical applications for cloning that will produce advances in medicine, biological research, and industry. Gene cloning, for example, is often used to study human disease. II OVERVIEW Farmers started cloning plants thousands of years ago in simple ways, such as taking a cutting of a plant and letting it root to make another plant. Early farmers also devised breeding techniques to reproduce plants with such characteristics as faster growth, larger seeds, or sweeter fruits. They combined these breeding techniques with cloning to produce many plants with desired traits. These early forms of cloning and breeding were slow and sometimes unpredictable. By the late 20th century scientists developed genetic engineering, in which they manipulate deoxyribonucleic acid (DNA), the genetic material of living things, to more precisely modify a plant's genes. Scientists combine genetic engineering with cloning to quickly and inexpensively produce thousands of plants with a desired characteristic. Cloning techniques can also be applied to animals. Scientists generate genetically modified animals with new traits, such as the ability to resist disease, and they use cloning techniques to reproduce these genetically modified animals. In the near future scientists hope to bolster populations of endangered species by cloning members from existing populations. Someday scientists may even resurrect extinct species by cloning cells from preserved specimens. Industry also utilizes cloning technology. For example, some bacteria eat toxic substances, such as gasoline or industrial chemicals, that are common pollutants. These bacteria can be cloned to make legions of bacteria with the ability to clean up environmental contamination (see Bioremediation). Likewise, cloned animals can be used to make a variety of ingredients, such as proteins, that are used in many commercial products. Perhaps most important from a human perspective, cloning promises great advances in medicine. Scientists have already inserted fragments of DNA containing the human gene for a blood-clotting protein into cells of a sheep. Through cloning techniques, scientists have generated new sheep whose milk contains the protein, which is needed by people with the blood-clotting disorder known as hemophilia. In the near future, researchers hope to use cloning to develop animals with human diseases and use these cloned animals to test the safety and effectiveness of new treatments devised for humans. Biomedical scientists hope to take cells from an ill patient, genetically modify them, and clone the modified cells to grow exactly the cells that the patient needs to regain health. Some scientists even imagine a day when cloning could be part of a process that grows entire organs for transplants. Despite the current and potential benefits of cloning, the process fuels a fiery battle. Little controversy ever surrounded plant cloning. In fact, few people even think of making plants from cuttings as cloning at all, but it is. Many people fight against the creation and cloning of genetically modified plants. But that worry generally involves the manipulation of the plant's DNA, not the cloning process. Animal cloning, on the other hand, stirs heartfelt controversy. Critics argue that the science of cloning is in its infancy and, in order to achieve success, mistakes may be made along the way. This could result in the development of cloned animals or humans with serious defects. Opponents to human cloning argue that without proper regulation, cloning could result in such questionable practices as designing babies with chosen genetic qualities so that they are more athletic, beautiful, or intelligent. Others fear that cloning tampers with God's will. As a result of so much controversy, the future of cloning remains uncertain. III CLONING IN NATURE Despite all of the modern concerns over cloning in laboratories, cloning started in nature. Many organisms replicate themselves through the process of asexual reproduction. Genetic information is encoded and transmitted from generation to generation in DNA, a coiled molecule organized into structures called chromosomes within cells. Segments along the length of a DNA molecule form genes. In asexual reproduction, an organism's DNA copies itself, and a new body grows around one copy of the DNA to form an offspring that is genetically identical to its parent. Organisms composed of just one cell, such as bacteria, reproduce through a type of asexual reproduction called fission. During fission, a cell duplicates its DNA to form two complete sets of DNA. This parent cell then divides to form two daughter cells. Each daughter cell receives one set of DNA. The two newly formed daughter cells are genetically identical to each other and to the parent cell. In another type of asexual reproduction known as budding, used by yeasts and some multicellular animals such as hydra, a little bump grows on the parent cell. The parent's DNA duplicates and a copy goes into the bump. The bump grows and eventually splits off as a clone of the parent. Many plants--including strawberries and some grasses--clone themselves by producing runners, which are stems that grow on top of the ground. A runner forms roots at any place that it hits the ground. The roots form a new plant that is genetically identical to the original. Other plants--including ferns, irises, and some grasses and trees--produce underground stems called rhizomes that generate new plants genetically identical to the parent plant. Some animals, including aphids, brine shrimp, and some species of fish, frogs, and lizards, reproduce asexually through a process called parthenogenesis--a word that derives from the Greek words parthenos ("virgin") and genesis ("birth"). In parthenogenesis a female's egg develops without fertilization from male sperm. Sometimes mammals also produce clones, but unlike the clones of other organisms the resulting offspring arise from sexual reproduction, in which a father's sperm fertilizes a mother's egg. In such cases, a mammal's fertilized egg divides in the womb and forms two or more embryos. These offspring are clones of each other, sharing exactly the same genes. They are not clones of the mother or father, however, since the offspring only have half of their genes in common with either parent. IV HOW SCIENTISTS CLONE CELLS Scientists initially made cloned cells in the laboratory by letting a single cell divide into a population of genetically identical cells. In this process scientists put the original cell in a laboratory dish containing culture medium (nutrients needed to keep a cell alive). The cell's natural process of mitosis (cell division) then produces genetically identical offspring. This process mimics how cells multiply, for instance, in plants and in the human body. Scientists later developed more complex cloning techniques using animal embryos. Every cell in an animal arises from a fertilized egg. The fertilized egg divides to form an embryo, and each cell in the embryo has the same genetic makeup. At some point in the embryo's growth and development, cells differentiate and become specialized. For instance, a heart cell only functions in the heart and not the liver, even though the genes of a heart cell and liver cell are the same. In the 1950s scientists began to experiment with embryo cells that were undifferentiated--that is, they had not yet specialized into a particular type of cell. Scientists found that such embryo cells are totipotent (able to give rise to all the different cell types in the body). Exploiting this characteristic, scientists developed three techniques to clone embryo cells: blastomere separation, blastocyst division, and somatic cell nuclear transfer. A Blastomere Separation In blastomere separation, scientists fertilize an egg cell with a sperm cell in a laboratory dish. The resulting embryo is allowed to divide until it forms a mass of about four cells. Scientists remove the outer coating of the embryo and place it in a special solution that causes the individual cells of the embryo, known as blastomeres, to separate. Scientists then put each blastomere in culture, where it forms an embryo containing the same genetic makeup as the original embryo. Each new embryo can then be implanted into the uterus of a surrogate mother to develop during a normal pregnancy. B Blastocyst Division In blastocyst division, scientists allow a fertilized egg to divide until it forms a mass of about 32 to 150 cells, known as a blastocyst. Scientists then split the blastocyst in two and implant the two halves into the uterus of a surrogate mother. The two halves develop as identical twins. C Somatic Cell Nuclear Transfer While blastomere separation and blastocyst division produce animals containing the genetic material from both a mother and father, somatic cell nuclear transfer produces an animal carrying the genetic material of only one parent. In this technique, scientists transfer the genetic material from a donor's somatic cell (any body cell other than an egg or sperm cell) to an enucleated egg cell--that is, an egg cell with its nucleus, and thus its genetic material, removed. The resulting cloned cell contains the genetic material of the donor's somatic cell. Scientists merge the somatic cell and enucleated egg cell using fusion or injection. In the fusion method, scientists place a somatic cell in contact with an enucleated egg cell. An electric pulse applied to the two cells pushes the somatic cell's nucleus into the enucleated egg cell. With the injection method, scientists inject the somatic cell's nucleus directly into the enucleated egg cell. In early experiments with somatic cell nuclear transfer, the procedure only worked using the nuclei from embryonic cells or cells from immature animals. In 1996 British scientists produced the sheep Dolly using a variation of somatic cell nuclear transfer that used the nuclei from adult cells. Scientists treated the adult donor cell to make it quiescent (less active) so that the genes of the adult cell behaved more like an undifferentiated embryo cell. They then isolated an udder cell from an adult sheep and starved the cell, forcing it into a resting stage that prevented the nucleus from dividing. Scientists found that this resting stage helps the adult cell return to an embryonic state. Scientists transferred the genetic material from the nucleus of the adult udder cell to an enucleated egg cell from a second sheep. The resulting embryo was then implanted into the uterus of a third sheep, where it developed during a normal pregnancy. The birth of Dolly paved the way for cloning cells taken from adult animals, enabling scientists to choose the mature individual they want to duplicate. Using cells from immature animals makes it more difficult for scientists to predict with certainty the physical characteristics of the resultant clone. Somatic cell nuclear transfer only uses genetic material found in a donor cell's nucleus. But not all of an animal's genes are located in the nucleus. A few dozen genes reside in the mitochondria, a cell structure found outside of the nucleus in the cell's cytoplasm. As a result, clones derived from somatic cell nuclear transfer may have mitochondrial genes from the enucleated egg cell used in the cloning process, not just genes from the donor's genetic material. In addition, since every organism is influenced by the interaction of both genes and the environment, cloned organisms may exhibit certain characteristics that differ from the genetic donor. A cloned animal, for instance, inevitably experiences many environmental factors during its development that differ from the parent's experience. These factors may include the types and quantity of food available, exposure to infectious diseases, or even the position of the embryo as it develops in the surrogate mother's womb. V STEM CELLS AND CLONING As part of the cloning process, scientists coax embryos to divide and grow. When embryos reach the blastocyst stage (around 32 to 150 cells), the embryos contain cells that can transform into any cell type that an organism needs during its development, such as blood cells, skin cells, and all the specialized cells that make up body tissues. Scientists can isolate these cells and encourage them to divide under special laboratory conditions to form embryo stem cells, which have the ability to form any cell type. Although all cells can divide to make copies of themselves, only stem cells can create new cell types. Humans maintain populations of stem cells in some tissues until death, but as most stem cells age they tend to lose their ability to transform into as many types of cells. The exception seems to be adult stem cells derived from the bone marrow, which maintain their ability to transform into many cell types. Biomedical scientists hope to harness the versatility of stem cells to fight disease. They theorize that if a patient receives stem cells cloned from a fertilized egg containing the patient's genetic material, the patient's immune system would not reject the stem cells as foreign material (see Medical Transplantation). These personalized stem cells could then be used to treat the patient's illness. In Parkinson disease, for example, specific brain cells die. One day, scientists hope to inject stem cells in the brain to rebuild the lost populations of brain cells. Such stem-cell injections might also be used to treat spinal-cord injuries, in which nerve cells in the spine have been destroyed, causing paralysis. Stem cells could also be used to repair damaged heart muscle after a heart attack or rebuild new cartilage in the joints of someone suffering from arthritis. Stem cells could even be grown to make cells from the pancreas that excrete insulin, and injections of such cells could cure diabetes mellitus. Medical procedures using stem cells still remain experimental. In 2001 the first clinical trial that injected stem cells into the brains of patients suffering from Parkinson disease produced mixed results. Although the injected cells grew, the treatment produced no obvious benefits for patients aged 60 and older. Some of the patients under age 60 said they felt better after the treatment, but about 15 percent of these younger patients acquired irreversible side effects, including twitching and other uncontrollable movements. Cloned stem cells could pose other risks. For example, the cloning process--producing large numbers of cells from one starting cell--could create genetic errors in the cells. If something went wrong in cell division during cloning, the error could be replicated in many other cells--even all of them if the error existed in the original cell. Nevertheless, in 2002 scientists at Rutgers University found few genetic mutations in embryonic stem cells cloned from mice. In fact, the study's investigators found those stem cells were better able to resist mutation than some adult cells. Some scientists worry that cloned stem cells could carry disease. For example, when cloning stem cells, scientists typically mix human stem cells with mouse cells in culture. The mouse cells produce an as yet unidentified nutrient or growth factor that helps keep the human stem cells alive. Scientists worry that infected mouse cells could just as easily transfer viruses to the human stem cells. They hope to develop new methods of cell culture that do not rely on such "feeder cells." VI CLONING ANIMALS Since the cloning of Dolly the sheep in 1996, scientists have cloned a wide variety of mammals from adult cells, including cows, goats, pigs, cats, and rabbits. While scientists have achieved some remarkable advances in animal cloning, drawbacks remain. Somatic cell nuclear transfer is inefficient--few cloned embryos survive through birth. For example, in experiments to create the first cloned rabbits in 2001, scientists implanted 371 embryos into surrogate mothers, but only six cloned rabbits were born. Perhaps more troublesome, early studies provide questionable results about the health of animals cloned using somatic cell nuclear transfer. As Dolly the sheep aged, scientists reported that she prematurely developed arthritis. Other experiments found that cloned mice suffered more illnesses and died at about half the age of normal mice. Animal cloning may have broader consequences. Healthy plant and animal populations in the wild maintain genetic diversity--a wide variety of genes in different combinations--through sexual reproduction. Genetic diversity can make a population more resistant to disease and environmental changes. Large populations of cloned plants or animals, on the other hand, may lack genetic diversity. Farmers around the world already grow many cultivated crops, including corn and rice, that have less genetic variability than their counterparts that grow in the wild. A single virus could wipe out these crops. Similarly, if every cow in a herd came from a single clone, one disease could potentially stamp out the entire herd. A Benefits of Animal Cloning Despite these drawbacks, scientists believe that animal cloning will one day advance agricultural practices and medicine, and even prevent the extinction of endangered animals. In agriculture, cloned cattle could produce a higher yield of meat or milk. The pharmaceutical industry already uses cloned animals to produce drugs for human use. For example, PPL Therapeutics in Scotland has generated sheep that produce milk containing a protein that helps in the treatment of hemophilia. One day pharmaceutical firms may clone large populations of genetically modified animals to quickly and inexpensively derive this protein for use in drug products. Cloned animals could also improve laboratory experiments. Researchers could create many genetically identical animals to reduce the variability in a sample population used in experiments, making it easier for scientists to evaluate disease. Moreover, scientists could clone a large number of animals that suffer from a human disease, such as arthritis, to study the disease's progression and potential treatments. Some cloned animals such as sheep and pigs live for years, and scientists could use these animals to evaluate their long-term response to drug treatments. B Cloning Endangered and Extinct Animals Cloning may bring some animal populations back from the brink of extinction. In 2001 scientists successfully cloned a gaur, an endangered ox that lives in Southeast Asia. Scientists inserted the genetic material from the skin cell of a dead male gaur into a cow's egg cell that had its nucleus removed. The resulting embryo was then implanted into a female cow, which served as the surrogate mother. The gaur calf died two days after birth from a bacterial infection apparently unrelated to the cloning process. Also in 2001 scientists cloned the mouflon, an endangered sheep from Sardinia, Corsica, and Cyprus. Teams of scientists also hope to clone other endangered animals, including the African bongo antelope, Sumatran tiger, and giant panda. Scientists may one day clone extinct animals. The last wild Spanish ibex, also known as bucardo, a mountain goat native to the Pyrenees mountain range of northern Spain, died in 2000. Spanish scientists preserved some of its cells, hoping to use the cells to create a cloned embryo and then implant the embryo into a more common type of goat with a genetic makeup similar to that of the Spanish ibex. In order for scientists to clone endangered and extinct animals, however, they need cells containing an intact nucleus with undamaged DNA. They also need to implant a cloned embryo into a surrogate mother from a closely related species. These requirements prevent scientists from cloning cells from the fossilized remains of dinosaurs and other long-extinct animals. C Can Humans Be Cloned? If scientists can clone animals, can they clone humans? In 1998 a South Korean research team announced that it cloned a human embryo through somatic cell nuclear transfer, but the embryo only survived to four cells. In 2001 researchers at the biotechnology firm Advanced Cell Technology claimed to clone human embryos that divided to six cells before dying. Many scientists argue that because the embryos from these two experiments did not double their cell size every 24 hours, they could not be considered true human embryos. In 2004 a team of South Korean researchers, led by Woo Suk Hwang of Seoul National University, announced that they had cloned human embryos capable of reaching the blastocyst stage and had succeeded in extracting stem cells from one of the embryos. However, a university panel later determined that Hwang's research was fabricated. Hwang resigned his position from the university. Many observers believed that stem cell research was likely to receive a major setback in funding as a result of the fraud. Although the research was fabricated, it still raised a number of ethical issues. To some people it is ethically unacceptable to destroy a human embryo for any reason. To others it is acceptable to do so if there is the prospect of understanding and treating human diseases. The production of embryos using the cloning method offers specific advantages. Embryonic stem cells could be matched to a patient so that tissues or organs developed from the embryo would be recognized by the patient's immune system. Otherwise, the patient's immune system would reject any foreign tissue or organs, or the patient would have to take drugs to suppress the immune system, which could lead to infections. However, this cloning procedure could be used to produce children who would be genetically identical to the person who donated the embryonic stem cell. A great majority of people would find this totally unacceptable. To many people it is important to draw a distinction between cloning to derive cells for therapeutic treatment and cloning to produce a child. VII WHY CLONING IS CONTROVERSIAL New areas of science often raise questions about safety. Early experiments in animal cloning attracted attention over its potential dangers. In some experiments in the early 1990s, for example, cloned cows developed faulty immune systems. Other projects created cloned mice that grew obese. In some studies, cloned animals seemed to grow old faster and die younger than normal members of the species. In 2002 the National Academy of Sciences released a report calling for a legal ban on human cloning. The report concluded that the high rate of health problems in cloned animals indicates that such an effort in humans would be highly dangerous for the mother and developing embryo and is likely to fail. Beyond safety, the possibility of cloning humans also raises a variety of social issues. What psychological issues would result for a cloned child who is the identical twin of his or her parent? How will a cloned child deal with the pressures of being compared to its genetic donor? A clone will never be identical to the genetic donor because environmental differences will influence the clone's development. Still, a cloned boy created from basketball star Michael Jordan's genetic material, for example, could suffer considerable criticism if he decided to pursue classical piano instead of slam-dunking. Are these issues compelling enough to ban the cloning of humans? Although some scholars argue that a clone might face unique problems, most offspring face some sort of burden. Children from poor families, for example, suffer some hardships that children from wealthy homes never imagine. Children in some developing nations face a tougher life than children in the United States. Nevertheless, few people would encourage a ban against having babies because of financial status or where a person lives. Cloning proponents argue that human cloning should not be banned simply because of potential hardships for the offspring. If human cloning ever becomes an option for parents, financial status could play a role because cloning would probably be expensive and only available to the wealthy. Accordingly, wealthy families might use cloning to give their offspring the best characteristics imaginable. Scientists could use genetic engineering to put together genes for such characteristics as beauty or intelligence, and then clone the cell to make a super child of sorts. If that capability was only available to wealthy people, the divide between the wealthy and the poor could widen farther than ever imagined. Soon after the cloning of the first human embryos in 2001, the Roman Catholic Church condemned such research. Many other religions agree that human cloning should be entirely and forever banned. Theologians view cloning as a thorny issue, an example of the ongoing tension between faith and science. Some people believe the scientific advances that enable human cloning are a God-given blessing. Others argue that scientists should not presume to play God by manipulating human genetic makeup. Some opponents claim that cloning must be forbidden because it involves destroying human embryos--such as the ones used to harvest stem cells. These opponents argue that any embryo is a viable human being and should never be destroyed intentionally. The research of the South Korean scientists in 2004, for example, came under criticism for precisely this reason. In order to extract stem cells from one of the cloned embryos, the embryo had to be destroyed. The South Korean research was also criticized because the female volunteers who donated their egg cells were given risky fertility drugs. VIII REGULATION OF CLONING These religious viewpoints, however, do not end the discussion over whether banning is right or wrong, good or bad. In all likelihood, the future of cloning in the United States and in other nations will depend on political actions. For instance, in August 2001 President George W. Bush used his executive powers to ban the use of federal funds for research on new stem cells derived from human embryos. This ban halted federally funded scientists from cloning new human stem cells from embryos, but it allowed them to continue using stem cells already developed. This ban did not prevent privately funded scientists from pursuing research on embryonic stem cells from humans. While Bush's policy dictated how government money would be spent on stem cell research, in the United States no federal legislation exists that regulates cloning, although a number of bills that restrict or ban cloning have been introduced to Congress. For example, in July 2001 the U.S. House of Representatives voted in favor of a bill outlawing any sort of human cloning. According to this bill, any scientist participating in human cloning--whether federally or privately funded--would face ten years in prison and a $1-million fine. This bill would also make it illegal to import any product developed from human cloning. In other words, even if scientists in another country developed a wonder drug through a process that involved human cloning, it could not be used in the United States. A similar bill passed the House in 2003. Both bills, however, failed to pass the Senate where there is support for the use of cloned stem cells in research or therapy. Other nations have sought to regulate human cloning, with varying degrees of restriction. France and Italy have banned human cloning altogether, and Russia's government approved a five-year moratorium on all human cloning research in 2001. The United Kingdom passed the Human Reproductive Cloning Bill in 2001, which "prohibits the placing in a woman of a human embryo which has been created otherwise than by fertilization." But British law permits scientists to create human embryos for research purposes. In Australia, the Gene Technology Act 2000 prohibits trying to clone humans or make human-animal hybrids. Many countries have yet to create laws against human cloning. Canada's government, for example, has tried twice to pass a law against human cloning, but both attempts failed. IX HISTORY OF CLONING Laboratory cloning techniques using undifferentiated embryo cells were first developed in the late 1800s, when German zoologist Hans Dreisch separated a sea urchin embryo when it was just two cells, and both cells grew to adults. In the early 1900s, German embryologist Hans Spemann extended Dreisch's work to salamanders. In his experiments Spemann determined that a nucleus from a salamander embryo cell could direct the development of a complete organism. He published his results in 1938 and proposed a "fantastical" experiment to produce an animal by removing the nucleus from one cell and placing it into an egg cell with its nucleus removed. A Early Frog Experiments In 1952 Spemann's proposed experiment became reality when American biologists Robert Briggs and Thomas King used cell nuclear transfer to insert DNA from a frog embryo cell into an enucleated frog egg. The resulting embryo grew into an adult. These early cloning experiments using cell nuclear transfer were successful only when the donor DNA was taken from an embryonic cell. In 1962 British developmental biologist John Gurdon began cloning experiments using nonembryonic cells--specifically, cells from the intestinal lining of tadpoles. Gurdon believed that the tadpoles were old enough so that cells taken from them would be differentiated. Gurdon exposed a frog egg to ultraviolet light, which destroyed its nucleus. He then removed the nucleus from the tadpole intestinal cell and implanted it in the enucleated egg. The egg grew into a tadpole that was genetically identical to the DNA-donating tadpole. But the tadpoles cloned in Gurdon's experiments never survived to adulthood and scientists now believe that many of the cells used in these experiments may not have been differentiated cells after all. Nevertheless, Gurdon's experiments captured the attention of the scientific community and the tools and techniques he developed for nuclear transfer are still used today. The term clone (from the Greek word kl ?n, meaning "twig") had already been in use since the beginning of the 20th century in reference to plants. In 1963 the British biologist J. B. S. Haldane, in describing Gurdon's results, became one of the first to use the word clone in reference to animals. B Cloning Mammals Scientists soon turned their attention to cloning mammals, which proved even more complex than earlier cloning experiments on invertebrates and amphibians. In 1977 German developmental biologist Karl Illmensee reported cloning mice from cells derived from early embryos. But Illmensee's findings were largely discredited because he used questionable laboratory techniques. Many agricultural researchers tried to clone cattle using somatic cell nuclear transfer, but it was not until 1984 that Danish biologist Steen Willadsen, working at Cambridge University in England, created the first cloned mammal. Willadsen cloned sheep by using nuclear transfer with DNA from early embryonic cells. Two years later, a team of researchers at the University of Wisconsin cloned a cow through a similar approach. In the 1990s cloning techniques advanced rapidly. In 1995 British scientists Keith Campbell and Ian Wilmut at the Roslin Institute cloned two lambs, named Megan and Morag, from embryonic cells. In this experiment, the scientists were able to keep the embryonic cells alive in culture for some time before beginning the cloning procedure. This advance enabled scientists to modify an embryonic cell's genes in culture before cloning it to produce genetically modified livestock. C The Birth of Dolly Scientists then began to focus their efforts on cloning a mammal with donor DNA from an adult cell. Scientists at the Roslin Institute succeeded in 1996 when the cloned sheep Dolly was born. Dolly came from a cell taken from an udder of an adult Finn Dorsett sheep and an enucleated egg from a Scottish blackface ewe. Dolly's birth proved that adult cells could acquire the cloning potential of embryonic cells. Like other efforts in cloning, however, this work demanded perseverance--it took 277 tries at somatic cell nuclear transfer to create Dolly. A parade of mammalian cloning followed Dolly. In July 1997 investigators at the Roslin Institute announced the birth of Polly, the first genetically modified lamb. In producing Polly, scientists inserted fragments of DNA containing the human gene for blood clotting factor IX, used in the treatment of hemophilia, into cells of a sheep. The animal cells incorporated the human DNA into their own genetic makeup. Scientists then cloned these cells to create Polly, who secreted the protein in her milk. Scientists hope to create a herd of Polly clones to replace traditional pharmaceutical methods for making this drug. In 1997 researchers at the University of Hawaii reported cloning more than 50 identical mice from an adult cell. Other scientists created a growing list of clones, including cows, goats, monkeys, pigs, and rabbits. In 2002 scientists cloned the first cat (named "CC," for carbon copy or courtesy copy), opening the door for the cloning of favorite pets. Despite these successes, animal cloning failures far exceed successes--in most experiments less than 5 percent of cloning efforts result in live clones. Reviewed By: Ian Wilmut Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.