Similar to luminal A and B subtype, as defined by GEP (Perou et al., 2000), the three receptor-based immune histochemical/compatibility (IHC) evaluation stratifies BC into ER+/PR+ HER2+, ER−/PR− HER2+, ER−/PR− HER2−, and ER+/PR+ HER2−, where all HER2+ cases shared similar genetic variations (Perou et al., 2000; Cheang et al., 2009) and outcomes (Network, 2012), irrespective of their hormonal subsets (Figure 2B). Here, ESR1 is linked to breast cancer.