ESR1 and breast carcinoma: Strikingly, breast cancer cells in direct co-culture with an immortalized human breast CAF cell line engineered through in vivo conditioning by MCF7 cells (CAF2, CAF:BC ratio 3:1) [11] exhibited a significantly muted ER-α activity following stimulation with estrogen (E2) in all three breast cancer cell lines, compared to mono-cultures of malignant cells (Fig. 2A–C).