ESR1 and breast carcinoma: Thus, to test whether antiestrogens can be leveraged for treatment of ER+/RB-deficient breast cancer, we treated MCF-7 and T47D, both WT and RBKO cells with fulvestrant (a selective ERα degrader) or by switching them to estrogen-free media to mimic estrogen suppression with aromatase inhibitors as is done in patients.