Mouse mammary tumor virus: a virus that exploits the immune system

Mouse mammary tumor virus (MMTV) causes mammary carcinomas and T-cell tumors in mice. MMTV variants that induce T-cell tumors have a large deletion within the U3 region of the long terminal repeat (LTR) compared to MMTV strains that induce mammary tumors. We provide evidence here that T-cell tropic MMTV strains lack a redundant binding site for a cellular protein called NBP (negative regulatory element binding protein). The lack of NBP-binding sites in T-cell tropic MMTV strains presumably leads to higher levels of transcription in T-cells during the MMTV life cycle and an increased incidence of mutagenic integration events.