Studies on the molecular biology of tumors have shown that these subtypes have different well-defined genomic profiles [3,4], which allows for subdividing BC into luminal A (ER-positive and PgR-positive/HER2-negative with lower-grade features), luminal B (ER-positive and/or PgR-positive but with higher-grade features or HER2-positive), HER2-positive (ER-negative/PR-negative/HER2-positive), and basal-like (ER-negative/PR-negative/HER2-negative). The gene discussed is ESR1; the disease is breast cancer.