ERBB2 and breast cancer: However, among 212 patients treated with chemotherapy and Trastuzumab, the AG or GG genotypes were associated with better DFS than the AA genotype (5-year DFS, P = 0.008), suggesting that the HER2 655 A > G polymorphism influences the HER2 gene function in HER2-positive breast cancers, with the G variant carriers having an aggressive phenotype but showing sensitivity to Trastuzumab.