We used mathematical models to analyze the age-incidence curve of breast carcinoma for individuals carrying a germline mutation in the BRCA1 or BRCA2 gene locus. Although many genomic abnormalities have been identified in breast tumors, we found that a two-stage model fit the data well. A one-hit model was not, however, consistent with the data. The results supported the hypothesis that the first hit represents loss of the wild type BRCA1 or BRCA2 allele as this occurs at a rate very similar to that for loss of the wild-type RB allele in retinoblastoma. Loss of the wild-type BRCA1 or BRCA2 allele appears to destabilize the genome as the second event occurs at a much higher rate. The second event is "rate limiting" in the sense that its occurrence is constrained by the limited number of intermediate cells with doubly mutated BRCA1 or BRCA2 alleles. The second event may not be unique, however. Loss of the wild-type BRCA allele appears to result in an increased rate for subsequent genomic events. A second event increasing proliferation of the partially malignant intermediate clone may lead inexorably to production and selection of cells with additional mutations in genes that facilitate tumor progression.