A newly published article has reported the presence of retroviral sequences similar to mouse mammary tumor virus in 40% of sporadic (nonfamilial) breast cancers in U.S. women and an even higher proportion in cases of inflammatory breast cancer and gestational breast cancer. Mouse mammary tumor virus is a retrovirus, namely, a virus containing ribonucleic acid (RNA) rather than deoxyribonucleic acid (DNA) as its genetic blueprint. A retrovirus uses its RNA to synthesize DNA in a process that enables genetic material from the retrovirus to become a part of the genes of an infected cell permanently. Mammary tumor virus sequences were not detected in normal breast tissue or other tumors. An entire proviral structure (the retrovirus RNA, capable of being incorporated into and subsequently replicate with the genome of the host cell) was found in two tumors. Breast cancer cells in culture were shown to contain and shed betaretroviral particles (betaretrovirus is the genus of retrovirus to which mouse mammary tumor virus belongs).

The virus found in women was designated human mammary tumor virus (HMTV) by the authors, who have investigated the presence of HMTV sequences in a number of breast conditions and geographic locations. Inflammatory breast cancer in U.S. women was found to have a higher incidence of viral sequences (71%) than other sporadic breast cancers. Similar incidences have been found in inflammatory breast cancers from Tunisia in North Africa, and in breast cancers diagnosed during or shortly after pregnancy. Since both of these conditions typically represent highly aggressive cancers, the authors conclude that HMTV is sometimes associated with a particularly malignant phenotype.