Evidence in favor of the existence of human breast cancer virus.

The existence in some women of a virus that is related to the mouse mammary tumor virus is evidenced by the following. Type B particles are occasionally found in human milk. A complication is the fact that most human milks destroy or damage the murine, mammary tumor virus (MuMTV) and its RNA-directed DNA polymerase (RDDP) when the mouse virus is added to the human milk. Human milk presumably has the same effect on any human RNA virions present. RDDP is found in many human milks although because of the destroying factors (not present in mouse or cow's milk), its presence usually depends on the freshness