A new study has reported that mouse mammary tumor virus-like DNA sequences were found in human milk in a small fraction of normal lactating women. Mouse mammary tumor virus (MMTV), which is found in wild mice and is also present in certain strains of mice used in breast cancer-related experiments, causes mammary tumors to develop in mice. This virus has been a long-standing candidate as a contributing cause of some breast cancers in women. Electron microscopic images of mouse mammary tumor virus-like particles were identified in milk from 5% of cancer-free lactating women 40 years ago. However, these observations have not been confirmed by modern methods to date. The study included 91 healthy lactating women from whom fresh breast milk samples were collected. The authors performed immunohistochemistry analyses on breast milk cells, as well as analyses on DNA extracted from the milk.

Mouse mammary tumor virus-like envelope gene sequences were found in 5% of the breast milk samples. These observations were verified by immunohistochemistry and DNA analyses. The authors conclude that the study findings confirm the presence of mouse mammary tumor virus-like gene sequences in human milk of some women. Mouse mammary tumor virus is transmitted in milk from mouse mothers to their newborn pups to cause mammary tumors when they become adults. It is possible that transmission of this virus occurs in humans similarly, according to the authors.