Broadly, BC sub-categories include the luminal ER-positive [luminal A (ER+, PR+, HER2−) and luminal B (ER+, PR+ with nuclear protein Ki-67 expression, HER2+/−)], HER2 positive (ER−, PR−, HER2+), and the triple-negative (ER−, PR−, HER2−) [15]. The gene discussed is ESR1; the disease is breast cancer.