standing disease development and progression, for regenerative medicine, and for targeted drug development.

In addition to that research accomplishment, the cloning of Dolly the sheep in 1997 using a technique called somatic cell nuclear transfer (SCNT) or, more simply, nuclear transfer (NT) provided a means of generating ES cells with defined genetic makeup. hES cell preparations could potentially be produced by using NT to replace the nucleus of a human oocyte, trigger development, and then isolate hES cells at the blastocyst stage. The advantage of using NT to derive hES cells is that the nuclear genomes of the resulting hES cells would be identical with those of the donors of the somatic cells. One obvious benefit is that this would avoid the problem of rejection if cells generated from the hES cells were to be transplanted into the donor. A more immediate benefit would be facilitation of a wide array of experiments to explore the underpinnings of genetic disease and possible forms of amelioration and cure. Some such experiments will not be possible using hES cells derived from blastocysts generated by in vitro fertilization (IVF), in which the nuclear genomes are not defined. Although the promise of using NT for such research is as yet unrealized, most researchers believe that it will be a critical source of both important knowledge and clinical resources. Use of NT for biomedical research, as distinct from its use to create a human being, has been considered by several advisory groups to be ethically acceptable provided that such research is conducted according to established safeguards against misuse and has undergone proper prior review. However, there is nearly universal agreement that use of NT to attempt to produce a child should not be allowed at present. The medical risks are unacceptable, and many people have additional objections to using this procedure for attempts at human procreation.

hES cells currently can be derived from three sources: blastocysts remaining after infertility treatments and donated for research, blastocysts produced from donated gametes (oocytes and sperm), and the products of NT. Ethical concerns about those sources of hES cells—combined with fears that the use of NT for research could lead to its use to produce a child—have fostered much public discussion and debate. In addition, concern has been expressed about whether and how to restrict the production of human/nonhuman chimeras in hES cell research. Research using chimeras will be valuable in understanding the etiology and progression of human disease and in testing new drugs, and will be necessary in preclinical testing of hES cells and their derivatives.

Because there is widespread agreement in the international scientific community about the potential value of hES cell research, the volume of this research has expanded since 1998, despite restrictions in the United States. First, federal legislation forbids the use of federal monies for any research that destroys an embryo; this effectively prevents any use of federal funds to derive hES cells from blastocysts. Second, research with established hES cell lines is limited by a policy announced by President George W. Bush in 2001 that restricts federal funding to research con-



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