Gene expression (mRNA) profiling studies have allowed intrinsic classification of breast cancer into five main subtypes: Luminal A (ER+/PR+/HER2−) are usually of low grade, luminal B (ER+/PR−/+/HER2+/−) are generally of higher grade with higher proliferation rate, normal-like subtype that resembles normal breast tissue and is associated with good prognosis, triple-negative breast cancer (ER−/PR−/HER2−), and HER2-enriched subtype (ER−/PR−/HER2+) [13,14,15]. The gene discussed is ERBB2; the disease is breast cancer.