In our meta-cohort of ER+, ERBB2 non-amplified cases of ILC and IDC, we found that ERBB2 mutations are enriched ILC, cluster in the functional kinase domain of HER2, and robustly associate with adverse clinical outcomes—independently of known prognostic clinicopathological features including LN status and tumor grade. The gene discussed is ERBB2; the disease is neoplasm.