Observations on the Genetics of Susceptibility for the Development of Mammary Cancer in Mice**

As inbred strains of mice have been under observation for more than 5 ~ generations of brother-to-sister matings, they should be satisfactory for genetic studies on the inherited susceptibility to cancer of the breast. In an effort to determine how this susceptibility might be transmitted, reciprocal matings were made between mice of the high mammary tumor A and the low mammary tumor B (C57 black) strains. Breeding females of the B stock have an incidence of mammary tumors of o. 5 per cent as reported by Little, Murray and Cloudman (9); in the author's line no mammary tumors have been observed in several hunmice of the ist and 2nd generations were mated i n t e r se to produce the animals of the 2nd and 3rd generations respectively. The total number of hybrids descended from mothers of the low cancerous strain was 734 and in no generation did the incidence of mammary cancer exceed 2 per cent. First generation hybrids with mothers from the cancer-susceptible strain and fathers from the low tumor strain had a high incidence of mammary tumors (ABF1, 95 per cent). In mice of the ABF., generation the incidence was 76.3 per cent (3oo mice).