A-T heterozygotes seem to have an increased risk but its size is unknown

Genetic predisposition accounts for 5-10% of breast cancer, and two genes—BRCA1 and BRCA2—have attracted most attention as high risk factors.1However, these two genes probably account for only a small proportion of the genetic risk while other more common but less penetrant genes may explain the remainder of genetically predisposed breast cancers.2 One such candidate is the gene, ATM, mutated in the human genetic disorder ataxia-telangiectasia (A-T).3 A-T heterozygotes (estimated to be 1% of the population) do not show any of the major symptoms of the disease, though there is good evidence that they have an underlying cellular radiosensitivity, but to a lesser extent than observed in A-T homozygotes.4These observations, together with earlier epidemiological studies, reveal a raised incidence of mortality from cancer among blood relations of patients with ataxia-telangiectasia, with the greatest relative risk for breast cancer (5.1) in female relatives of patients.5

An association between the incidence of breast cancer and A-T heterozygosity was also revealed in two separate but smaller studies. 6 7 Based on an independent assessment …