Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis

Many mammalian viruses have acquired genes from their hosts during their evolution1. The rationale for these acquisitions is usually quite clear: the captured genes are subverted to provide a selective advantage to the virus. Here we describe the opposite situation, where a viral gene has been sequestered to serve an important function in the physiology of a mammalian host. This gene, encoding a protein that we have called syncytin, is the...