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<p>Abstract</p> <p>The known breast cancer susceptibility genes only account for 20% to 25% of the excess familial risk of the disease <abbrgrp><abbr bid="B1">1</abbr></abbrgrp>. The present study assessed the contribution of <it>BRCA1/2 </it>mutations and <it>CHEK2 </it>variants to the relative risk of breast cancer for women with affected mothers or sisters. The familial relative risks were estimated by Poisson regression based on the Swedish Family-Cancer Database. The Database was also used to calculate the distribution of life expectancy, the number of daughters per family and the age specific cumulative risk of female breast cancer. This information, together with the penetrances of <it>BRCA1, BRCA2 </it>and <it>CHEK2 </it>from the literature, was used to simulate the familial clustering of breast cancer under different scenarios. The excess risk explained by <it>BRCA1, BRCA2 </it>and <it>CHEK2 </it>decreased steeply with the age at diagnosis of the cancers. Around 40% of the familial risk for cases diagnosed before the age of 50 years was associated with <it>BRCA1/2 </it>mutations. In contrast, roughly 85% of the familial risk of breast cancer diagnosed before the age of 69 years remained unexplained. The contribution of <it>CHEK2 </it>to familial breast cancer was small.</p>