  Most HESCs are derived from embryos that were created for infertility treatment but that were in excess of what the infertile individual(s) ultimately needed to achieve a pregnancy. The HESCs derived from these leftover embryos offer investigators a powerful tool for understanding the mechanisms controlling cell differentiation.  However, there are scientific and therapeutic reasons not to rely entirely on leftover embryos. From a research standpoint, creating embryos through cloning technologies with cells that are known to have particular genetic mutations would allow researchers to study the underpinnings of genetic diseases in vitro. From a therapeutic standpoint, the HESCs obtained from leftover IVF embryos are not genetically diverse enough to address the problem of immune rejection by recipients of stem cell transplants.  (Induced pluripotent stem cells may ultimately prove sufficient for these research and therapeutic ends, since the cells can (a) be selected for specific genetic mutations and (b) provide an exact genetic match for stem cell recipients.)  At present, the best way to address the therapeutic problem is through the creation of a public stem cell bank that represents a genetically diverse pool of stem cell lines (Faden et al. 2003, Lott &amp; Savulescu 2007). This kind of stem cell bank would require the creation of embryos from gamete donors who share the same HLA-types (i.e., similar versions of the genes that mediate immune recognition and rejection).