Protein encoded by proviral genes of endogenous retroviruses. When a retrovirus infects a host cell, viral reverse transcriptase (RT) makes a DNA copy of the RNA viral genome. The integrated DNA form of a retrovirus is referred to as a provirus. Proviral genes are expressed by cellular mechanisms. Retroviruses that enter the germline are referred to as endogenous retroviruses (ERVs) to distinguish them from horizontally transmitted, not passed on to host progeny, "exogenous" retroviruses. Amplification of ERV copy number via retrotransposition or reinfection has given rise to numerous ERV sequences in the vertebrate genomes. As much as 8% of the human genome, and 10% of the mouse genome, consists of sequences derived from ERV insertions.