This heterogeneity leads to the classification of BC into four distinct subtypes: luminal A (ER+ and/or PR+, HER2−), luminal B (ER+ and/or PR+, HER2+), HER2 (HER2+, ER−, PR−), and triple-negative (ER−, PR−, HER2−). The gene discussed is ERBB2; the disease is breast cancer.