Breast cancers are typically classified into seven subtypes (3): luminal A (ER positive and histologic low grade), luminal B (ER positive and histologic high grade), HER2 overexpressing, basal-like (2 types – BL1 and BL2), immunomodulatory (IM), mesenchymal (M), mesenchymal stem-like (MSL) [3] and normal breast-like tumours [1]. The gene discussed is ESR1; the disease is breast cancer.