Proposed molecular subtypes [9-11] include: 1) Luminal A [12,13]: ER+ and/or PR+, HER2-, Ki67 < 14%; 2) Luminal B: ER+ and/or PR+, HER2-, Ki67 ≥ 14%; 3) Luminal BH: ER+ and/or PR+, any Ki67, HER2 overexpressed or amplified; 4) ERBB2/HER2 overexpression: ER-/PR-, HER2/neu overexpressed or amplified; [14] and 5) Basal-like [15]: ER-/PR-/HER2- (also called triple negative breast cancer, TNBC; most BRCA1-mutant breast cancers are basal-like TNBC) [16]. The gene discussed is ESR1; the disease is breast cancer.