ESR1 and breast cancer: Knowing that ER is a valuable target for BC therapy, to evaluate the effect of ATRA on MCF-7 ER-positive cell lines, which act as hormone-dependent cells, and to highlight the importance of ER isoforms in modulating H19 and hTERT expression, cells were treated with 5 μM ATRA inhibitor and/or with 100 nM fulvestrant for 48 h.