Gallen 2013 consensus, BC molecular subtypes are defined according to estrogen receptors (ERs), progesterone receptors (PRs), Human Epidermal Growth Factor Receptor-2 (HER2), and the proliferation marker Ki67 expression as per the following: luminal A-like (ER+/PR+, HER2−, Ki67+ < 20%); luminal B-like HER2− (ER+/PR+ < 20%, HER2−, Ki67+ ≥ 20%); luminal B-like HER2+ (ER+/PR+, HER2 overexpression); HER2 overexpressed (non-luminal (ER−, PR−); HER2 overexpression); and basal-like and/or triple-negative BC (TNBC) (ER−, PR−, HER2−) [12] (Figure 1). The gene discussed is ESR1; the disease is breast cancer.