Importantly, this molecular classification has successfully discovered sub-classes of ER-positive and/or PR-positive breast cancers as luminal A and luminal B. This is a significant achievement because even though clinical assessment of IHC utilizes ER, PR, and HER2 status, these markers could not let the separation of these two distinct subtypes which have very different clinical outcomes [3, 8]. Here, ESR1 is linked to breast cancer.