ESR1 and breast cancer: Studies conducted in ER+ tamoxifen-sensitive cells, such as MCF-7 and T47D, and tamoxifen-resistant cells derived from these cells, such as LCC2 and LCC9, showed that lncRNA UCA1 was upregulated in tamoxifen-resistant cells compared to tamoxifen-sensitive cells, and UCA1 confers tamoxifen resistance to breast cancer cells via the mTOR, Wnt/β-catenin, and PI3K/AKT signaling pathways [184,185,186].