In this single center study, the majority of patients (102/168; 60.7%) with pT1ab pN0 HER2-positive breast cancer received adjuvant systemic therapy with trastuzumab, including 16 (50%) pTmic patients, 34 (52%) pT1a patients and 52 (74%) pT1b patients. Here, ERBB2 is linked to breast carcinoma.