In this single-center, retrospective cohort of 79 patients with invasive breast carcinoma, paired immunohistochemical assessment of ER, PR, HER2, and Ki-67 on diagnostic core biopsies and surgical specimens confirmed a high overall level of concordance, but also documented a non-negligible subset of clinically relevant reclassifications. The gene discussed is ERBB2; the disease is invasive breast carcinoma.