Largely in agreement with the gene expression profiling data, breast cancer is categorized into HR+/HER2− (ER+, PR+ and HER2− or luminal-A), HR+/HER2+ (ER+, PR+ and HER2+ or luminal-B), HER2+ (HER2+, ER− and PR− or HER2-enriched), and triple-negative breast cancer (TNBC or ER−, PR− and HER2− or basal-like) subtypes by established immunohistochemistry (IHC) classifier [13]. The gene discussed is ESR1; the disease is breast cancer.