ESR1 and breast cancer: However, the basal-HER2+ phenotype was significantly associated with inferior OS by univariate analysis and, after accounting for strong prognostic variables such as tumor size at diagnosis in stepwise multivariate analysis, the presence of the basal-HER2+ phenotype, but not ER status, also predicted significantly worsened DFS when a positive CK5/6 staining in ≥ 10% of the HER2+ breast cancer tissue sections was registered as a diagnostically relevant positive reaction.