March 2024—Human epidermal growth factor receptor 2 is a critical biomarker in breast cancer, gastrointestinal malignancies, and other cancers. HER2 protein expression can be evaluated using IHC, and the DNA copy number of its encoding gene, ERBB2, can be evaluated using FISH. In most clinical settings, IHC evaluation is categorized as positive (3+), equivocal (2+), or negative (0 to 1+), with equivocal cases being reflexed to FISH. Patients with HER2-positive tumors, defined as either 3+ or 2+/FISH positive, have been eligible to receive HER2-targeted therapy for many years. More recently, the FDA approved the antibody-drug conjugate trastuzumab deruxtecan (T-DXd) to treat patients with HER2-low breast cancer, defined as tumors with IHC 1+ or 2+/FISH negative. This promising treatment has allowed many more patients to receive molecular-targeted therapy.