In heat maps of relative gene expression, samples were stratified by as previously described immunohistochemistry (IHC)-defined receptor status of ER, PR and HER2 expression, or by the PAM50 molecular classification (five intrinsic subtypes: luminal A (LumA), luminal B (LumB), HER2-enriched, basal-like, and normal-like breast cancer [2]) (Figure 1A). The gene discussed is ESR1; the disease is breast carcinoma.