ESR1 and breast neoplasm: Microarray‐based gene expression profiling of breast tumours identified at least five major intrinsic subtypes: Basal‐like, Luminal A, Luminal B, Human epidermal growth factor receptor 2‐positive/estrogen receptor‐negative (HER2+/ER−) and Normal Breast‐like (Prat and Perou, 2011).