Breast cancer is a complex, heterogeneous and multifactorial disease, and can be classified by the profile of gene expression or immunophenotyping in different intrinsic subtypes: Luminal A (ER+ PR+ HER2− Ki-67 < 14%), Luminal B (ER+ PR+/− HER2 ± Ki-67 ≥ 14%), HER2+ (ER− PR− HER2+ Ki-67 > 20%) and Triple Negative (TN) (ER− PR− HER2− Ki-67 > 30%)5,6. The gene discussed is ESR1; the disease is breast cancer.