  This approach [the introduction of human ES cells into a non-human blastocyst] is an obvious extension of techniques widely used in research with [mouse ES] cells—namely, aggregation of morulas from two mice or injection of mES cells into mouse blastocysts. In both cases, the cells can contribute extensively to any mouse that arises from implantation of such a chimeric blastocyst. Clearly, an animal (e.g., mouse) blastocyst into which human cells are transplanted raises other issues because potentially the inner cell mass, the progenitor of the fetus, would consist of a mixture of human and mouse cells. It is not now possible to predict the extent of human contribution to such chimeras. (NAS 2005, 34; for further discussion see Streiffer (2005, 355–357, 363–364).)