Inheritance of one defective copy of either of the two breast cancer susceptibility genes, BRCA1 and BRCA2, predisposes individuals to breast and ovarian cancers. Current progress in determining the function of these genes suggests that they participate in a common pathway to facilitate orderly homologous recombination and thereby maintain genomic integrity. As a consequence of this defect in homologous recombination, tumors that arise in BRCA carriers are likely to be more sensitive to ionizing radiation. This review summarizes recent investigations about the nature of the defect in DNA repair, and highlights the unanswered questions about the tumor suppressor paradox of BRCA genes. The unsolved mystery is the other genetic changes that must occur to turn a BRCA-deficient cell from a nonviable cell into a tumor cell capable of endless growth.