In this exploratory analysis of biomarker data from the phase III MARIANNE study, none of the evaluated biomarkers (all of which are related to the HER2 pathway) showed predictive value for PFS benefit with T-DM1 treatment alone or in combination with pertuzumab versus control in this group of patients with HER2-positive advanced breast cancer previously untreated in the advanced setting. The gene discussed is ERBB2; the disease is breast cancer.