Hereditary breast and ovarian cancer (HBOC) syndrome is a genetic disorder that is diagnosed by the detection of germline mutations in the BRCA1 or BRCA2 gene. BRCA1 and BRCA2 are critical in homologous recombination (HR) in conjunction with RAD51 recombinase. BRCA1/2 functions as a tumor suppressor gene, and its loss of function results in chromosomal instability, commonly observed in cancer. In vertebrates, HR is essential for survival, and HR deficiency caused by the mutations in the genes involved in the fundamental processes of HR generally leads to lethality at the cellular level. Since all proliferating cells with hypomorphic mutations in BRCA1/2 genes, irrespective of their origin, cause HR defects in any proliferating cells, it remains elusive why BRCA1/2 mutations selectively cause breast and ovarian cancers. This chapter discusses the association between BRCA1/2 mutations and organ-specific carcinogenesis based on the latest findings on BRCA1 and BRCA2 in DNA repair.