Human epidermal growth factor receptor 2 (HER2)-low breast cancer is a clinically relevant subgroup defined by low but detectable HER2 protein expression, immunohistochemistry (IHC) score of 1+ or 2+ with negative in situ hybridization findings, positioned at the interface between traditional HER2-positive and HER2-negative disease. The gene discussed is ERBB2; the disease is breast cancer.