We have previously detected an MMTV env gene-like 660 bp sequence in 38% of human breast cancers, but not in normal tissues or other tumors. In this communication we report the sequences from eleven tumors and three breast cancer cell lines, and compare them to four strains of MMTV and to the known endogenous retroviral sequences. The breast cancer sequences were highly homogenous to the MMTV's, but not to the endogenous sequences suggesting an exogenous origin.