  A chimera is an individual composed of cells with different embryonic origins. The successful isolation of five human embryonic stem cell (hESC) lines in 1998 increased scientists’ ability to create human/non-human chimeras and prompted extensive bioethics discussion, resulting in what has been dubbed “the other stem cell debate” (Shreeve 2005). The debate about chimeras has focused on five main arguments. The Unnaturalness Argument explores the ethics of violating natural species boundaries. The Moral Confusion Argument alleges that the existence of entities that cannot be definitively classified as either human or non-human will cause moral confusion that will undermine valuable social and cultural practices. The Borderline-Personhood Argument focuses on great apes and concludes that their borderline-personhood confers a high enough degree of moral status to make most, if not all, chimeric research on them impermissible. The Human Dignity Argument claims that it is an affront to human dignity to give an individual “trapped” in the body of a non-human animal the capacities associated with human dignity. Finally, the Moral Status Framework maintains that research in which a non-human animal’s moral status is enhanced to that of a normal adult human is impermissible unless reasonable assurances are in place that its new moral status will be respected, which is unlikely given the motivations for chimeric research and the oversight likely to be provided. These arguments provide different rationales for evaluating chimeric research and consequently differ in their implications both for the range of chimeric research that is unethical as well as the way chimeric research should be addressed in public policy.