Breast cancer heterogeneity: Breast cancer is biologically heterogeneous: current molecular classifications based on transcriptome-wide analysis, clinical determinations of steroid hormone receptor (like ER) status, human epidermal growth factor receptor 2 (HER2) status, or proliferation rate status (PR), all suggest a minimum of four distinct biological subtypes [47]. The gene discussed is ERBB2; the disease is breast carcinoma.