ESR1 and cancer: Based on the expression levels of the two most important receptors, HER2 and Estrogen Receptor (ER), these subtypes can be summarized as HER2+ cancers, characterized by amplification of HER2/neu on both ER+ and ER- background, ER+ HER2− cancers, characterized by the expression of ER at no HER2 background, and ER− HER2− breast cancers, when none of these receptors are expressed.