Or they can be genes-individually, collectively, or, via nuclear transfer, as whole genomes. They can be gametes (eggs and sperm), as in hybrids. In current uses they can be cells, tissues, or larger body parts. The biological constituents mixed vary substantially. The core idea in the biological use of chimera is captured by the following broad definition: "a single biological entity that is composed of a mixing of materials from two or more different organisms." This broad definition can then be played out across four important dimensions: I believe their broader approach is appropriate. Robert and Baylis go beyond all of these definitions, not only in the specifics of some of their proposed chimeras, unanticipated by dictionary writers, but by including hybrids. The OED traced the term to a German scientist, H.Winkler, in 1907. In its 1989 second edition the OED added, as the fourth figurative definition, the following: The Oxford English Dictionary (OED) did not give a biological definition of chimera in its 1971 edition, although the famous second edition of Webster's Unabridged Dictionary had already provided a botanical definition by 1934: I conclude that ethical issues are not raised by whether something is a chimera but on the basis of three other questions about the chimeric organism: its "humanity," its "naturalness," and its proposed uses. In this commentary, therefore, I first offer a taxonomy of chimeras and then speculate on how that taxonomy might illuminate the ethical issues the category raises. I believe that we can achieve a better understanding of the ethical issues raised by chimeras-and, indeed, whether the category "chimera" is useful in ethical discussion of contemporary biology-by defin- ing chimera more exhaustively and then examining the concerns associated with different types of chimeras. In their third paragraph Robert and Baylis list a broad set of possible types of chimeras before, in their fourth paragraph, focusing on human-to-animal embryonic chimeras. Did Bellerophon, riding Pegasus, slay a monster with the heads of three different species or a one-headed beast with parts from three species? This lack of clear definition exists in contemporary discussions of the ethics of nonmythological chimeras, including in the useful article by Jason Scott Robert and Françoise Baylis, "Crossing Species Boundaries" (2003). The original chimera turns out to be surprisingly undefined. The American Journal of Bioethics 3.3 (2003) 17-20
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