Induction of mammary tumors in C57BL/haag female mice by a low oncogenic mammary tumor virus

Mammary tumor virus (MuMTV)‐negative mouse substrain C57BL/Haag foster‐nursed by strain DD becomes a line with a moderately high incidence of mammary tumors occurring in the females at a late age. Electron microscopy revealed B particles in the tumors, providing additional evidence for the propagation and transmission of DD‐MuMTV in C57BL/HaagfDD. Since both albino strains DD and RIII originated in Europe, develop pregnancy‐dependent mammary tumors, carry black and agouti genes (cc, BB, AA), and therefore may be related, evidence is presented which suggests that the two naturally occurring MuMTVs from DD and RIII may also be related, and that they may share a common origin.