Background
Mouse mammary tumor virus (MMTV) has a proven role in breast cancer (BC) in susceptible murine lines. MMTV-related sequences were also found in significant proportion of human sporadic and familial BC patients (17-52%). MMTV-homologous sequences were found in liver, timus and spleen of wild commensal mice; in 63% animals analyzed the sequences were expressed [1–3]. However, the role and origin of MMTV-related infection in humans is unclear.