Molecular analysis of breast cancer based on the gene expression profile (e.g., estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2)) has enabled the classification of BC into different subtypes, such as luminal A, luminal B, HER2 and basal (or triple negative BC) types of breast cancers. The gene discussed is ESR1; the disease is breast carcinoma.