ESR1 and cancer: The ER+ HER2− cancers, which at large correspond to the “Luminal” breast cancers as identified by clustering of whole genome mRNA expression, can be further subdivided into so-called Luminal A, which are mostly low grade tumors with good prognosis, and Luminal B which are ER+ tumors of mostly high grade, with high expression of proliferative genes and poorer prognosis1, 2, 4.