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In the Spring of 2003, as the first article on the ethics of grafting human stem cells or their derivatives into non-human embryos was going to press (Robert and Baylis 2003), we hosted a small invitational meeting in Halifax, Nova Scotia to discuss the ethics of making human-nonhuman embryonic chimeras in stem cell biology. We were intrigued by, and wanted to further investigate, claims made by stem cell scientists that such research was necessary for at least two reasons: to create model organisms for studying human cells; and as a precursor to first-in-human clinical trials of stem cell transplantation. Henry Greely was invited to this meeting in his role as chair of the working group initiated by Dr. Irving Weissman to review the ethics of two proposed research projects involving the transplantation of human brain stem cells into postnatal and fetal mice. To our dismay, at the meeting, Greely indicated that he could not …