A subclassification that has recently gained considerable attention in breast cancer is the so called HER2-low group (HER2 IHC 2 + /ISH- or IHC 1 + ), based on the introduction of antibody-drug conjugates (ADCs) targeting HER2-expressing cells (e.g., the FDA-approved ADC Trastuzumab Deruxtecan). The gene discussed is ERBB2; the disease is breast carcinoma.