ESR1 and breast cancer: Over-expression of UCA1 acts as a molecular sponge and down-regulates miR-18a by associating with the Ago2-containing RNA-induced silencing complex (RISC) [21], supporting the idea that ER-positive breast cancer cells can acquire tamoxifen resistance via a UCA1-miR-18a-HIFα feedback loop (Figure 2) [20].