Current evidence strongly supports a role for the breast cancer susceptibility genes, BRCA1 and BRCA2, in both normal development and carcinogenesis. Valuable clues regarding the function of these genes have been garnered through studies of their patterns of expression. A central feature of the in vivo pattern of BRCA1 and BRCA2 expression is that each of these putative tumor suppressor genes is expressed at maximal levels in rapidly proliferating cells. This feature is consistent with in vitro observations that BRCA1 and BRCA2 are expressed in a cell cycle-dependent manner. This feature is also well illustrated during mammary gland development wherein the expression of BRCA1 and BRCA2 is induced in rapidly proliferating cellular compartments undergoing differentiation, such as terminal end buds during puberty and developing alveoli during pregnancy. Strikingly, the spatial and temporal patterns of BRCA1 and BRCA2 expression are virtually indistinguishable during embryonic development and in multiple adult tissues despite the fact that these genes are unrelated. These observations have contributed to the emerging hypothesis that these genes function in similar regulatory pathways.