ESR1 and breast carcinoma: Breast cancer tissue sections of 154 consecutive patients with HER2-overexpressing primary invasive ductal breast cancer were evaluated for expression of ER and CK5/6 by IHC: 89 (58%) tumors were luminal-HER2+, 39 (26%) tumors were HER2+, and 24 (16%) tumors were basal-HER+ (Table 1).