Abstract
The incidence of spontaneous breast cancer was tabulated for reciprocal F1 breeding [female][female] made by crossing high and low tumor strains of mice which had been inbred 25 and 11 generations respectively. In the outeross 36 hybrid [female][female] had mothers from the high cancer strain; and 91.7% developed breast, tumors at an average age of 10.4 months. None of the 10 hybrids from crosses made in the reciprocal direction had breast tumors; average age at death 22.5 months. Seven developed other types of tumors: fibrosarcoma; cavernous hemangioma; an undifferentiated carcinoma probably ovarian in origin; and four lymphoblastomas.

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