The majority of patients (102/168; 60.7%) with pT1ab pN0 HER2-positive breast cancer in our single center study received adjuvant systemic therapy with trastuzumab, including 16 pTmic patients (50%), 34 pT1a patients (52%) and 52 pT1b patients (74%). The gene discussed is ERBB2; the disease is breast carcinoma.