## RISK OF
BREAST AND OVARIAN CANCER IN BRCA1 GENE CARRIERS, UNDER THE
ASSUMPTION OF HETEROGENEITY

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 .