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Mechanisms of oncogenesis. LTRs recruit transcription factors from the infected cell for retroviral gene transcription. These LTRs can also enhance transcription of the host cell genes, leading to uncontrolled tumor cell proliferation. Some HERVs encode potentially oncogenic proteins like Np9 and Rec that interact with transcription factors and activate immunosuppressor pathways, promoting oncogenesis. HERVs can also induce chromosomal translocations in somatic cells that could lead to tumor proliferation. HERVs also can promote an immunosuppressive response that may lead to cancer formation and spreading, because the Env protein has an immunosuppressive domain (ISD).