Mouse Strain GR: A Comparative Study of Its Naturally Occurring Mammary Tumor Virus

The virulence of murine mammary tumor virus (MuMTV), transmitted by milk and gametes, from strain GR mice with high incidence of tumors was compared with milk-borne MuMTV's from strains C3H, A, and DD, also with high Incidences of mammary tumors, by foster-nursing of genetically similar susceptible BALB/c breeders and virgins. The incidence of mam: mary tumors in BALB/cfGR (with GR-MuMTV) breeders was similar to incidences for breeders of BALB/cfC3H (with C3H-MuMTV), BALB/cfA (with A-MuMTV), and BALB/cfDD (with DD-MuMTV), but the average age of tumor onset was later. However, when the MuMTV's were compared in virgin BALB/c females, GR-MuMTV was less effective than C3H- and A-MuMTV's and similar to DD-MuMTV in the production of tumors of the mammary gland.