Expression of DNA sequences homologous to sequences of env gene of mouse mammary tumor virus (MMTV) in the lymphocytes of patients with breast cancer and in subjects at a high risk of breast cancer has been reported. Antigen analogous to envelope protein gp52, product of MMTV env gene, is detected in T lymphocytes of virtually all patients with breast cancer and extremely rarely in T cells of controls, where its expression is confined to B cells. For explaining such unexpected results, we studied the molecular basis of this antigen synthesis. Specific PCR products were obtained using primers to gp52-coding region of MMTV env gene. One of them (957 nucleotides) was used as a probe for hybridization of DNA and RNA from lymphocytes of patients with breast cancer and controls. This sequence was hybridized with 90% frequency with genome DNA of breast cancer patients and with 85% frequency with genome RNA of such patients, which is almost 4-fold more than in the controls (patients with gynecological tumors or donors). These results correlate with the frequency of detection of the studied antigen in patients with breast cancer and control group patients.