ESR1 and breast cancer: Intriguingly, when we silenced AR endogenous expression with a pool of siRNAs (a mixture of 4 siRNA provided as a single reagent) (Figure 1E), upregulation of miR-9-5p was observed (compared to a siRNA negative control) but only in ER+ BC cell lines MCF-7 and T-47D, with no effects on the ER- cell line MDA-MB-453 (Figure 1F).