A 26-year-old woman has a lumps in her left breast. On physical examination, she has an irregular, firm, 2-cm mass in the upper inner quadrant of the breast. No axillary adenopathy is noted. A fine-needle aspirate of the mass shows anaplastic ductal cells. The patient's 30-year-old sister was recently diagnosed with ovarian cancer, and 3 years ago her maternal aunt was diagnosed with ductal carcinoma of the breast and had a mastectomy. Mutation involving which of the following genes is most likely present in this family?

Approximately 5% to 10% of breast cancers are familial, and 80% of these cases result from mutations in the BRCA1 and BRCA2 genes. The onset of these familial cancers occurs earlier in life than the sporadic cancers. The protein products of these genes are involved in DNA repair. BCL2 is overexpressed in some lymphoid neoplasms. The epithelial growth factor receptor ERBB2 (HER2) overexpression is present in some sporadic breast cancers; other EGF alterations can be seen in lung, bladder, gastrointestinal, ovarian, and brain neoplasms. The HST1 mutation is seen in some gastric cancers. IL2 overexpression is associated with some T cell neoplasms. K-RAS overexpression is seen in many cancers, including some breast cancers, but the early age of onset and family history in this case strongly suggest BRCA mutations.