A. ERα expression levels in different ER positive cells. Cell lysates of MCF-7, T47D, ZR-75-1, BT474, MCF-7:5C, and MCF-7:2A were harvested. MCF-7, T47D, ZR-75-1 and BT474 cells were cultured under conditions with estrogen (10% FBS), while MCF-7:5C and MCF-7:2A cells were cultured under estrogen-free conditions (10%SFS). ERα expression levels were examined by immunoblotting with primary antibody. Immunoblotting for β-actin was determined for loading control. B. Modulation of ERα expression in the absence of estrogen. Wild-type ER positive MCF-7, T47D, ZR-75-1, and BT474 cells were cultured under conditions with estrogen (10% FBS) or without estrogen (10% SFS) for 3 days, respectively. Cell lysates were harvested. ERα expression levels were examined by immunoblotting with primary antibody. Immunoblotting for β-actin was determined for loading control.