Abstract
Mammary tumorigenesis appears to be influenced by several factors including genetic background, hormones and the immune response. In animal models there is a correlation between infection with mouse mammary tumor virus (MMTV) and an increased incidence of spontaneous mammary tumors. Exogenous MMTV is transmitted from high mammary tumor incidence strains to certain low incidence strains through the milk (1) with the result of increased numbers of tumors in the latter. There is evidence suggesting that MMTV infection results in integration of this retrovirus into the mammary glandular tissues (2). In recent years the possibility has been explored, that expression of an endogenous counterpart of MMTV may be involved in the etiology of mammary tumors. The observation by Hageman et al. (3) of MMTV virions in mammary tumors arising in some low incidence mouse strains that had different characteristics from the classic exogenous milk transmitted virus, supports a role of endogenous MMTV in tumorigenesis.