ESR1 and breast cancer: In fact, the androgen receptor (AR) is expressed in the vast majority of breast cancers, with some studies reporting expression of AR in up to 90% of primary tumors and 75% of metastatic lesions [5,6], although more contemporary studies suggest that the frequency of AR expression varies depending on the subtype of breast cancer (for example, ERα-positive (luminal) versus triple-negative and basal breast cancers), and other clinical and pathologic parameters [7-9].