Breast cancer affects one out of every 10 women in industrialized countries, and is a leading cause of cancer morbidity and mortality in women. Ovarian cancer, although less common than breast cancer, is very difficult to treat effectively, in part due to difficulty in early diagnosis of the disease. Most cases of breast or ovarian cancer appear to occur without a clear family history of the disease. These sporadic cases account for approximately 95% of all breast cancer. Possibly, low-penetrance genes contribute as risk factors to this group of cancers. However, about 5% of breast cancers occur clustered within families. Importantly, cancer in these familial syndromes characteristically manifests at a younger age than sporadic cancer. Therefore, in terms of both the number of women affected and the effect of disease on individual families, the familial breast and/or ovarian cancer syndromes have a large impact upon society. Over the past few years, the genetic basis of familial breast and ovarian cancer has become clear. It is caused by germline mutations affecting one of two autosomal tumor suppressor genes, BRCA1 and BRCA2 . Surprisingly, these two genes have been found to participate in the control of homologous recombination, suggesting that they may function as tumor suppressors by regulating genome integrity maintenance functions.