A multivariate analysis conducted to evaluate the influence of patient and tumour characteristics on the decision to administer or not adjuvant chemotherapy in pT1a-pT1b breast cancers, showed that younger age, grading G3, high proliferative index, ER-negative status and HER2-positive status were significantly associated with the decision to administer adjuvant chemotherapy. The gene discussed is ERBB2; the disease is neoplasm.