ERBB2 and breast cancer: BRCA1 is an important tumor suppressor gene in that: (1) germline BRCA1 mutation contributes to approximately 20%–25% of familial breast cancer cases, (2) BRCA1 expression is markedly reduced in approximately 1/3 of sporadic breast cancer cases, and (3) the majority of BRCA1-deficient cancers are triple-negative breast cancer (TNBC; ER-, PR-, and Her2-), which has the worst prognosis among breast cancer subtypes due to the lack of effective therapies3–5.