Endogenous C-type viruses: double agents in natural life processes.

C-type RNA viruses have been described in about 20 different vertebrate species. Their presence in man is suggested by electron microscopic, biochemical and serologic studies, although a definite re;icating human virus has not yet been isolated. These viruses are inherited through the germ cell and their production is regulated by genetic information carried in the host cell (i.e. endogenous virus). Two classes of endogenous C-type viruses have been recognized in certain animals particularly the mouse: ecotropic and xenotropic. They may have their counterparts in man. Ecotropic viruses spread through the host and can be easily transmitted to cells of the same species; they can produce malignancy. Xenotropic viruses cannot infect cells from their host species but are infectious for cells from heterologous species. The interaction between xenotropic and ecotropic viruses could lead to the transfer among species of genetic information relating to normal life processes and malignancy. These C-type viruses may play a role in evolution, normal development and differentiation as well as autoimmune disease and cancer.