Background
Endogenous retroviruses are genetic parasites of mammalian genomes. They are « remnants » of ancestral infections by retroviruses that had integrated into the germ line of the host and were then transmitted vertically, in a Mendelian fashion. Studies of several endogenous retrovirus families have led to the identification of functional elements, able to replicate inside the host. Whereas some endogenous retroviruses behave as bona fide retroviruses, with a replicative cycle involving extracellular viral particles, some of them disclose a strictly intracellular amplification cycle with viral particles accumulating in the cytoplasm or in intracellular organelles.