Genetics and epidemiology of ataxia-telangiectasia.

: According to the Hardy-Weinberg principle, the frequency in the general population of heterozygous carriers of a gene causing an autosomal recessive syndrome in homozygotes is likely to be between 0.1% and 5%. It is thus important to know whether carriers of the AT gene have a risk of cancer or diabetes greater than comparable noncarriers. A retrospective study of blood relatives in 26 AT families, and follow-up of the obligatory heterozygotes in those families, demonstrated an excess of diabetes, deaths from cancer, and deaths from ischemic heart disease among obligatory or probable AT heterozygotes. Hypotheses about the disease-predisposing effects of the AT gene in the heterozygote are now being reexamined, retrospectively and prospectively, in almost 150 newly identified AT families. Specific tests for the AT gene will permit even more rigorous tests of these hypotheses.