A flood of comparative genomic data is resulting in the identification of human lineage-specific (HLS) sequences. As apes are our closest evolutionary relatives, transgenic introduction of HLS sequences into these species has the greatest potential to produce ‘humanized’ phenotypes and also to illuminate the functions of these sequences. We argue that such transgenic apes would also be more likely than other species to experience harm from such research, which renders such studies ethically unacceptable in apes and justifies regulatory barriers between these species and other non-human primates for HLS transgenic research.