We have reported previously (Wang et al, Cancer Res., 55: 5173-5179, 1995) the presence of a 660-bp sequence that is 90-98% homologous to the env gene of mouse mammary tumor virus in 38% of 314 unselected breast cancer samples, in some breast cancer cell lines, and in 1.8% of the 107 normal breast specimens from reduction mammoplasties. In this communication, we have investigated whether the 660-bp sequence or a smaller 256-bp sequence within the 660 bp were expressed in breast tumors, normal breasts, and breast cancer cell lines by means of reverse transcription-PCR. The results indicated that expression of the 660-bp sequence was detected in 66% of the sequence-positive breast tumors studied and in none of the negative tumors or normal breasts and that the 256-bp sequence was expressed in all of the env-positive breast cancer cell lines tested. Sequence analysis revealed that the expressed 660-bp sequence is 98% homologous to the env gene of mouse mammary tumor virus with only one small stretch of 46 bp homologous to endogenous viral sequences. The presence and expression of this sequence is related to a group of human breast cancers in which the concept of a retroviral etiology can be entertained.