A prognostically important differentiation hierarchy occurs among all breast cancers and is classified into Luminal-A (ER+, PR+/−, HER2−), Luminal-B (ER+, PR+/−, HER2+), Basal/Triple-negative (ER−, PR−, HER2−), HER2 (ER−, PR−, HER2+) and Claudin-low (ER−, PR−, HER2−) which are also referred to as intrinsic subtypes (Kennecke et al., 2010). Here, ESR1 is linked to breast cancer.