Comparison of the Number of Breast-Cancer Deaths Observed in Relatives of Breast-Cancer Patients, and the Number Expected on the Basis of Mortality Rates2

Grandmothers, mothers, aunts, and sisters of women with breast cancer have had breast cancer with a frequency which is significantly greater than that of women in a similar age range either in the general population or in 2 sets of selected control samples. This excess cannot be entirely environmental in origin, since it is found almost to the same extent in both paternal and maternal grandmothers and aunts. It cannot be attributed to biased selection or to chance distribution of the trait through the population, since the excess is significantly greater than can be accounted for by either of the above factors. Better recall by women with breast cancer, whose relatives are similarly affected, has no bearing on this excess, since the entire family of both control and breast-cancer samples was thoroughly investigated. The ratio of observed-to-expected number of breast cancers is higher in unmarried than in married aunts and sisters. The fact that the presumably genetic factor responsible for this excess of breast cancer is enhanced to the greatest degree in childless women shows the interaction of genetic and extrinsic factors.