In a Netherlands retrospective study of 3512 patients with stage I HER2-positive breast cancer, 385 (11%) patients had a T1a tumor (including 54 with micro-invasive disease ≤1 mm), and 800 patients (23%) with a T1b tumor had a 5% breast cancer specific survival improvement with systemic treatment in each of the three tumor size subgroups T1a, T1b, and T1c HER2-positive breast cancer.[11] To address in an exploratory way whether trastuzumab impacted adjuvant treatment benefit, we examined IDFS separately in the AST only (N = 47) and AST+T (N = 102) subsets compared with untreated patients. The gene discussed is ERBB2; the disease is breast carcinoma.