| Summary: | Like other cancer types, breast cancer is considered to be a genetic disease. While the majority of genetic changes are somatic, a minority are in germline. About 10-20% of breast cancer is thought to be due to a germline mutation in high-penetrance genes, where the major focus has been on BRCA1 and BRCA2. Some of these mutations are defined as founder mutations. Studies on founder mutations yield important information, mainly due to a large number of available carriers with the same mutation, regarding penetrance, expression, genetic modifiers or low-penetrant genes and influence from the environment. Population studies are also valuable due the possibilities for evaluating clinicopathological data in a group of patients who have the same mutation. In Iceland a rare founder mutation has been detected in BRCA1, and a frequent founder mutation has been detected in BRCA2. In addition to population-based studies on genetics and clinicopathology, an extensive analysis of somatic changes in tumours of BRCA2 founder mutation carriers has been made.
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