The presence of germline pathogenic variants of BRCA1 or BRCA2 is called hereditary breast and ovarian cancer (HBOC), and the lifetime incidence of breast, ovarian, and related cancers is known to be higher in patients with these variants than in the general population. Both the BRCA1 and BRCA2 are genes related to homologous recombination repair, and homologous recombination deficiency (HRD) is known to occur in the absence of these genes. However, there are differences in subtypes and the age at onset of breast cancer between BRCA1 and BRCA2 variant carriers. This chapter discusses the difference in age at the onset of breast cancer among carriers of BRCA1 and 2 pathogenic variants and the cause of this discrepancy.