Third, the smaller the number of cell lines in use, the lower the genetic diversity that they represent. A prohibition on the derivation of new cell lines might result in research that focuses on cell lines that are not optimal and might preclude the replacement of inferior materials with more efficient cell lines. Experience with other kinds of cells in culture has shown that cell lines can be expected to accumulate mutations that reduce their suitability and safety for research (Kunkel and Bebenek, 2000). There is little evidence that ESC lines will behave any differently.

Fourth, it has been suggested that it is biologically preferable to derive stem cells from embryos created specifically for research rather than from surplus embryos at in vitro fertilization clinics, although both employ similar techniques in the initial stages. Several ideas underlie that suggestion. Embryos from couples who have turned to in vitro fertilization because of infertility might have inherent, but as yet unrecognized, biological defects. From a broader genetic perspective, couples who seek treatment for infertility might not be representative of the genetic diversity of society as a whole. In addition, it might be preferable to obtain embryos that have not been frozen before stem cells are derived from them, inasmuch as freezing could have unexpected effects. Each of these concerns has only a theoretical basis, and there is currently little evidence with which to evaluate the relative merits of stem cells created specifically for research versus those derived from surplus embryos.

Given the many unanswered questions about the biology of stem cells, the successful development of new medical therapies depends in large part on the performance of an enormous amount of basic research. Basic research is defined as systematic study directed toward greater knowledge or understanding of the fundamental aspects of phenomena and of observable facts without specific applications, processes, or products in mind. Since World War II, basic research has been the