Scientists are considering creating a hybrid (or technically, a chimera) formed from mouse and human cells to be used as a host to test embryonic stem cells. Scientists are generally bad at considerations that touch on philosophy or ethics, so the following should give readers reason to pause:
Dr. Irving L. Weissman, an expert on stem cells at Stanford University, said that making mice with human cells could be "an enormously important experiment," but if conducted carelessly could lead to outcomes that are "too horrible to contemplate." He gave as an extreme example the possibility that a mouse making human sperm might accidentally be allowed to mate with a mouse that had made its eggs from human cells
Of course, there are other views. Take that of Dr. Fred Gage, who suggests "that the question of making mice with human cells deserved further consideration and that scientists and the public ’should listen to each other more’ before reaching a conclusion to go ahead. . . . The earlier the mice were killed the smaller would be the ethical issue, in his view."