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The National Academy of Sciences recently issued voluntary guidelines to govern human embryonic stem cell research. Among other restrictions, these guidelines prohibit certain kinds of combinations of human and nonhuman animal cells, and call for ethics review and oversight of any protocol involving the transfer of human embryonic stem cells into nonhuman animals. In this essay, I discuss the history of and scientific rationales for combining human cells with cells of nonhuman animals, and critically assess the most recent attempts to limit such research on moral grounds—and find them lacking. Nonetheless, as I show, this research remains scientifically and morally contested. I then explore whether and how the NAS's recommended Embryonic Stem Cell Research and Oversight committees will allow for scientifically well‐informed moral assessment of this controversial, but possibly important, research …