CYP19A1 and breast neoplasm: So, it's very important to determine whether AR promotes metastasis of ER+ breast cancer cells in the absence of E2 as a model of postmenopausal women with breast cancer (80% of ER+ breast tumors patients) treated with an aromatase inhibitor (AIs) [38] or in the presence of both E2 and DHT (where ER is fully active).