Figure 1. Overview of the ERV co-option process with immune system functions annotated.

Retroviral endogenization occurs when an exogenous retrovirus integrates into the genome of a host germline cell. The integrated element can be co-opted to act as a coding gene or a promoter region for a host gene. Three functions of co-opted ERVs in the immune system that are discussed in this review are annotated. Recent evidence has also suggested that ERV RNA transcripts, in addition to ERV-encoded proteins and ERV-derived regulatory regions, may also mediate antiviral immunity in vertebrates.