(endogenous retrovirus) Genetic elements propagated primarily through the germline, with a proviral structure, i.e. the chromosomally-integrated form of retroviruses and regulated by cellular processes. Could be involved in development and serve as progenitors to pathogenic retroviruses. Classified as ecotropic or xenotropic. Ecotropic viruses can be transmitted to cells of the same species and can produce malignancies. Xenotropic viruses cannot infect cells from their host species but are infectious in heterologous species. Cycles of transcription and reverse transcription make the elements mobile and mutagenic.

Retroviruses that have integrated into the germline (PROVIRUSES) that have lost infectious capability but retained the capability to transpose.