Studies of the Mammary-Tumor Agent of Strain RIII Mice

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Abstract
An inbred strain of RIII mice was established from mice obtained in 1945. During the first 27 generations of inbreeding, the mice showed a high incidence of mammary tumor at a low average age, but beginning with the F28 generation the incidence declined and the average age increased. Evidence suggests that the agent lost its effectiveness and, in some instances, to such a degree that it either disappeared from or became inactive in a few mice; descendants of these mice developed few mammary tumors. The occurrence of tumors in reciprocal F1 hybrids from strains C3H and RIII revealed that the RIII agent was the weaker. When agent-free C3H females were suckled by RIII foster mothers, the fostered mice and their progeny developed few breast tumors, but there was no unequivocal evidence that the RIII agent had disappeared from strain C3H mice. When presumably agent-free RIII mice were suckled by strain C3H foster mothers, the fostered mice developed tumors. Almost all the descendants of 3 fostered RIII mice also developed tumors at low average ages. The mice descended from another fostered RIII female showed a high incidence of tumors in the first 4 generations of inbreeding. Mice of the F5 generation of this line and of the following 4 generations have shown a low tumor incidence; only 1 of 31 mice developed a tumor and the remaining mice were tumor-free at 6 to 23 months of age at the time of writing this paper. This is good evidence that the C3H agent can either disappear from or become inactive, at least temporarily, in strain RIII mice.