Retroviruses infect the host, from which new genes are often acquired, and subsequently use the function of these genes. It is well known that one of retroviruses, Rous sarcoma virus (RSV) acquired an oncogene, src, from the host. If a retrovirus uses the function of a gene from the host, a host organelle could also acquire new genes from retroviruses. Vertebrate animals possess many genomic sequences that might have been acquired from retroviral genes (Endogenous Retrovirus, ERV). Evidence so far accumulated indicates that the expression of genes, which might have been acquired from retroviruses, is very high in the placenta. It is therefore speculated that retroviral genes are actively involved in placental formation and, possibly, functioning. This review proposes a working hypothesis based on recent findings and developments associated with syncytins of retroviral origins and their functions in trophoblast differentiation/placental formation.