Sorlie et al. utilized gene expression profiling (GEP) to create a distinctive molecular portrait of breast cancer using 456 cDNA clones, and reclassified tumors into five intrinsic clinical subtypes: Luminal A (ER+ and/or PR+/HER2-), Luminal B (ER+ and/or PR+/HER2+), Basal-like (ER- and/or PR-/HER2-), HER2+ (ER- and/or PR-/HER2+), and normal-like tumors [15, 16]. The gene discussed is ESR1; the disease is breast carcinoma.