One of the unique features of retroviruses is their ability to integrate their genetic information in the genomes of their host cells, including the germ line, and to persist there as so-called proviruses. Proviruses which are contained in the germ line of a given species and are inherited from generation to generation like cellular genes are called endogenous retroviruses (for review see 1). Although the function or bioiological role of endogenous retroviruses still remains to be elucidated, they have been detected in almost all vertebrate species examined. The most relevant properties of endogenous, genetically transmitted retroviruses are summarized in Table 1. Endogenous retroviruses persist in cellular DNA, are transmitted through the germ line, and possess a transposon-like structure (2) which enables them to integrate at any position of the cellular genome. Endogenous retroviruses can be activated by certain chemicals such as mutagens/carcinogens or mitogens, by radiation, and by other mechanisms such as DNA-viruses or physiological processes (e.g. aging) to express antigens or to form infectious virus particles. Their biological relevance is unknown but may include involvement in physiological processes such as protection against superinfection by related retroviruses, similar to observations made with exogenous retroviruses in some animal model systems.