Human breast cancer is a heterogeneous disease and may be subdivided based on the expression of estrogen receptor (ER, ER-positive, ER+; ER-negative, ER-), HER2 and pathologic grade: Luminal A (ER+, low grade), Luminal B (ER+, higher grade), HER2-positive (either ER+ or ER-) and Basal-like (ER-/HER2-) [10]. The gene discussed is ESR1; the disease is breast cancer.