Development and characterization of the BALB/cNIV mouse strain

The strain BALB/cNIV/Crgl was developed by infecting BALB/c/Crgl mice with mouse mammary tumor virus from C3Hf mice. A BALB/c normal mammary duct was transplanted into the gland-free fat pad of a hormone-stimulated female C3Hf X BALB/c F1 mouse. A hyperplastic alveolar nodule was found in the BALB/c ductal outgrowth and was transplanted into another hybrid gland-free fat pad. The resultant hyperplastic alveolar outgrowth was finally transplanted to female BALB/c mice. The hyperplastic alveolar outgrowth contained an exogenous, infectious mouse mammary tumor virus named the nodule-inducing virus, which was thought to be derived from the endogenous low oncogenic mouse mammary tumor virus found in C3Hf mice. The hyperplastic alveolar outgrowth-bearing BALB/c mice were inbred for four generations, and one family was selected as the strain BALB/cNIV/Crgl. It was found that (a) the mouse mammary tumor virus found in the BALB/cNIV strain was milk transmitted, but not transmitted by infected males; (b) the BALB/cNIV breeding females had a low tumor incidence (40%) and a longer latent period (14 months) than did female BALB/cfC3H mice (92% at 8 months); (c) the BALB/cNIV nodule outgrowths had low tumor-producing capabilities (50%) and longer latent periods (13.4 months) than did nodule outgrowths derived from female BALB/cfC3H mice (100% at 7.7 months).