ESR1 and breast cancer: To identify breast cancer risk-associated lncRNAs that regulate cell proliferation, CRISPR-Cas13d-based knockdown screens were performed in a normal mammary epithelial cell line [12] (K5+/K19+) and two breast cancer cell lines (estrogen receptor, ER-positive MCF7 cells and ER-negative MDAMB231 cells; Fig. 1a).