ESR1 and breast cancer: Indication for 21-gene expression assay testing includes ER-status as a dichotomous parameter, and it is well described that MBC are more likely to develop ER-positive breast cancers than their female counterparts: In their analysis of available ER data from the Surveillance, Epidemiology and End Results database, Anderson et al. established that 92.4% of MBC are ER-positive (2575/2788) compared to 77.5% of female patients (344,406/444,558) [25].