Endogenous retroviral sequences present in the human genome

: The human genome contains a variety of elements resembling mammalian retroviruses. Most of these sequences are related to primate, murine C-types viruses, murine B-type viruses and A-type particles or HTLV. These sequences comprise at least 0.1-0.6% of human genome. They are transcriptionally active especially in human placenta, embryonic tissue and in human tumor cell lines. Endogenous retroviruses and retroviral elements represent a cellular reservoir of possibly pathogenic retroviral genes.