MMTV was formerly known as Bittner virus, and previously the "milk factor", referring to the extra-chromosomal vertical transmission of murine breast cancer by adoptive nursing, demonstrated in 1936, by John Joseph Bittner while working at the Jackson Laboratory in Bar Harbor, Maine.

Several mouse strains carry the virus endogenously, but it is also transmitted vertically via milk from mother to pup.

The viruses become transported through the gastrointestinal tract to the Peyer's patches where they infect the new host's macrophages, and then lymphocytes.

[citation needed] The mouse mammary tumor virus (MMTV) has formerly been classified as a simple retrovirus; however, it has recently been established, that MMTV encodes an extra self-regulatory mRNA export protein, Rem, with resemblance to the human immunodeficiency virus (HIV) Rev protein, and is therefore the first complex murine retrovirus to be documented.

If the virus is transferred exogenously, it is passed from the mother mouse to her pups through her milk.

[citation needed] Endogenous MMTV reacts to the whole range of hormones that regulate normal mammary development and lactation, response has been demonstrated to steroid hormones (androgens, glucocorticoids and progestins),[13] as well as prolactin.

[14] When the mouse reaches puberty the virus begins to express its messenger RNA in the estrogen sensitive tissues.

[12] The LTR (long terminal repeat) of MMTV contains a glucocorticoid hormone response element.