ERBB2 and metastatic neoplasm: Tumors with HER2-low (IHC 1+ or 2+/ISH−)—and the emerging HER2-ultralow subset (IHC 0 with faint, incomplete staining in ≤10% of cells)—are clinically negative for traditional agents but may access certain HER2-directed ADCs, particularly in metastatic disease; eligibility for ultralow is evolving [4].