The vast majority of HR+ breast tumors are characterized as human epidermal growth factor receptor 2 (HER2) negative; however, approximately 60% of them are defined as HER2 low, characterized by low levels of HER2 expression with immunohistochemistry (IHC) expression 1+ or 2+ and negative in situ hybridization (ISH) for HER2 gene amplification [26]. The gene discussed is ERBB2; the disease is breast neoplasm.