ESR1 and breast cancer: To this end, we engineered three ER-α-expressing human breast cancer cell lines with a luciferase reporter construct under the control of three consecutive Estrogen Response Elements (ERE) to generate ER-α reporter cell lines: MCF7 (ER-α+; PR+), T47D (ER-α+; PR+), and BT474 clone 5 (ER-α+; PR- ; HER2+).