"Human stem cell research is controversial not because of its goals, but rather because of the means of obtaining some of the cells. Research involving most types of human stem cells, such as those derived from adult tissues and umbilical cord blood, has been uncontroversial, except when its effectiveness as an alternative to embryonic stem cells is debated. The crux of the debate centers around embryonic stem cells, which enable research that may facilitate the development of medical treatments and cures, but which require the destruction of an embryo to derive. In addition, because cloning is one method of producing embryos for research, the ethical issues surrounding cloning are also relevant."
CRS Report for Congress, RL33554
U.S. Department of State, Foreign Press Centers: http://www.fpc.state.gov/