A new set of guidelines for human embryonic stem cell research issued by the National Academies of Sciences would permit the creation of human-animal hybrids, but would not allow them to breed. The guidelines permit the introduction of human embryonic stem cells into nonhuman mammals, "under circumstances where no other experiment can provide the information needed." Private research on human embryonic stem cells should be guided by responsible practices, and oversight committees should be established to ensure these practices are followed, say the National Academies' National Research Council and Institute of Medicine in the new set of guidelines.