ERBB2 and cancer: HER2-directed antibody, antibody-drug conjugate and small molecule tyrosine kinase inhibitor-based treatment regimens are effective for patients with HER2+ (over-expressing) cancers, defined as a score of 3+ by immunohistochemistry (IHC; scale of 0–3 + ) and/or ERBB2 copy number (CN) elevation by in situ hybridization (ISH; HER2/CEP17 ratio ≥2.0 or CN ≥ 6.0 signals/cell), and in some cases less effective for brain metastases owing to the blood-brain tumor barrier1–3.