MCF-7:5C, MCF-7:2A, and MCF-7:RAL representing LTED estrogen-independent BC were treated with 1 μM E4, 1 μM BMI-135, and a combination of these with 1 μM 4OHT and 1 μM endoxifen to investigate whether E4 and BMI-135 exert their function via ERα. This evidence concerns the gene ESR1 and breast cancer.