Endogenous proviruses.

Sequences related to different retroviruses are present in the mammalian genome, being inherited through the germ line, and some of these sequences are expressed as RNA and protein products. The ubiquitous presence of these viral sequences suggests that they are related to some essential cellular functions. However, these functions remain to be defined. The possible role of endogenous provirus expression in cell differentiation and proliferation as well as in tumorigenic processes remains enigmatic. Available evidence strongly suggests that endogenous retroviruses present in the human and nonhuman mammalian genome are not oncogenic.