ESR1 and breast cancer: We therefore knocked down PTEN expression, using a small hairpin RNA (shRNA) mir-based system (Fellmann et al., 2013) (PTEN KD), or knocked-out PTEN expression using CRISPR-Cas9 (PTEN KO), in two ER+PIK3CA-mutant breast cancer cell lines, T47D (H1047R) and MCF7 (E545K) (Figures S2A and S2B).