Cancers in 44 families with ataxia-telangiectasia.

Cancer incidence was measured retrospectively in 574 close blood relatives of white ataxia-telangiectasia (A-T) patients and 213 spouse controls in 44 previously unreported families. The cancer incidence rate in the adult blood relatives was significantly elevated over the rate in the spouse controls (rate ratio = 3.9, p less than 0.01). For heterozygous carriers of the A-T gene, the relative risk of cancer was estimated to be 6.1 (p less than 0.005) as compared with nonheterozygotes. The most frequent cancer site in the blood relatives was the female breast, with nine cancers observed. These findings provide further support for the hypothesis that heterozygotes for the A-T gene are predisposed to cancer.