ESR1 and breast cancer: In breast cancer cells (MCF-7 and MDA-MB-231), its combination with 5-fluorouracil (5-FU) enhances cytotoxicity by inducing caspase-dependent apoptosis, reducing ER-α protein levels, and suppressing the ER-α/Akt signaling axis, thereby potentiating the pro-apoptotic effects of 5-FU [19].