CGIN1: a retroviral contribution to mammalian genomes.

This study describes the origin and structural features of a mammalian gene, CGIN1 (Cousin of GIN1). CGIN1 proteins contain an NYN domain, retroviral RNase H and integrase domains, and a domain of unknown function (CGIN1 domain) that is also present in two other genes (N4BP1 and KIAA0323). We suggest that CGIN1 derives from the fusion of a KIAA0323-like gene with retroviral sequences, which occurred prior to the marsupial-eutherian split. Sequence and structural analyses indicate that the CGIN1 integrase domain is inactive but still retains the 3D folding observed in retroviral integrases. We hypothesize that CGIN1 may contribute to retroviral resistance in mammals by regulating the ubiquitination of viral proteins.