Taken from Easton et al. (1995). Data from 33 families, each containing at least 4 individuals with ovarian or breast cancer diagnosed before age 60, and each having a posterior probability of linkage to BRCA1 of at least 90%. Two different susceptibility alleles were assumed, (1) and (2), conferring different breast and ovarian cancer risks. The major difference between the two alleles is in the estimated ovarian cancer risk. Hence allele 1 is estimated to confer a breast cancer risk of 62% by age 60 years and an ovarian cancer risk of 11%, while allele 2 confers a breast cancer risk of 38% and an ovarian cancer risk of 42%. Under this model, the population frequency of allele 2, as a proportion of all susceptibility alleles, was estimated to be 0.29 (95% confidence limits 13%- 45%). This model is a highly significantly better fit than is the homogeneity model .