> We have reviewed the evidence relevant to mouse mammary tumour viruses (MMTV) and human breast cancer. The prevalence of MMTV- like gene sequences is 15-fold

> We have reviewed the evidence relevant to mouse mammary tumour viruses (MMTV) and human breast cancer. The prevalence of MMTV- like gene sequences is 15-fold higher in human breast cancer than in normal human breast tissue controls and is present in up to 40% of human breast cancers. > The evidence with respect to MMTV being responsible for a subset of approximately 30–40% of human breast cancers in Western countries and 10–20% in Asian countries is compelling. An exception is the lack of detailed information about causal mechanisms. A role for MMTV-like viruses in human breast cancer is highly likely.