Fig. 1.

Evolutionary origin of human endogenous retroviral genes (HERVs). Retroviruses can infect somatic cells of the host and the virus can then spread in the population (horizontal transmission). Retroviral infection of host germ cells potentially results in the integration of viral DNA into the host genome. Integrated sequences display typical proviral structure with two LTR flanking both ends and gag, env, pol viral genes. In the case of germ cell infections, these sequences can be vertically inherited in Mendelian fashion (vertical transmission) and preserved in some form during evolution to provide a selective advantage. Mutations in proviral sequences have caused these viruses to become unfunctional. However, in rare cases during evolution, the ORFs of some of the viral genes have become conserved and adopted several physiological functions, mainly in placenta formation. LTR, long terminal repeat; SUPYN, suppressyn. Created in biorender.com