ESR1 and cancer: It is a heterogeneous disease divided into five major subtypes by DNA microarray gene expression profiling: luminal A (estrogen receptor [ER]+, progesterone receptor [PR]±, HER2−), luminal B (ER+ , PR± , HER2+), HER2-enriched (ER−, PR−, HER2 +), basal-like (ER−, PR−, HER2−), and normal-like cancers [3, 4].