| Summary: | <p>Abstract</p> <p>Penetrances of <it>BRCA1 </it>and <it>BRCA2 </it>mutations have been derived from retrospective studies, implying the possibility of ascertainment biases to influence the results.</p> <p>We have followed women at risk for breast and/or ovarian cancer for two decades, and report the prospectively observed age-related annual incidence rates to contract breast or ovarian cancer for women with deleterious <it>BRCA1 </it>or <it>BRCA2 </it>mutations based on 4830 observation years. Patients were grouped according to mutation, age and having/not having had previous cancer.</p> <p>In women not having had previous cancer and aged 40-59 years, the annual incidence rate to contract breast or ovarian cancer in those having the most frequent <it>BRCA1 </it>founder mutations was 4.0%, for women in this age group and with less frequent <it>BRCA1 </it>mutations annual incidence rate was 5.9%, and for women with <it>BRCA2 </it>mutations 3.5%.</p> <p>The observed figures may be used for genetic counseling of healthy mutation carriers in the respective age groups. The results may indicate that less frequent <it>BRCA1 </it>mutations have higher penetrances than <it>BRCA1 </it>founder mutations.</p>
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