ESR1 and breast carcinoma: Because AR is co-expressed with ER in up to 90% of ER+ breast cancer [38] and AR can recapitulate the ER-mediated transcriptional program seen in luminal breast cancers [39], we applied the AR-ness signature to tumors that are ER- by IHC, to identify ER- tumors with apocrine features, active AR signaling, and/or expressing androgen-induced genes.