Breast Cancer Cell Line Cultivation and Manipulation

T-47D and MCF-7 cells were purchased from American Type Culture Collection (ATCC). T-47D cells were cultured in RPMI medium (Sigma-Aldrich) and MCF-7 cells were cultured in DMEM medium (Sigma-Aldrich) supplemented with 10% fetal bovine serum (Genesee Scientific) and 2% Penicillin–Streptomycin (Sigma-Aldrich) in a humidified incubator with 5% CO₂. Generation of ESR1 mutant (Y537S and D538G) MCF-7 and T-47D cells were previously described^{8 (link),23 (link)}. Tamoxifen-resistant MCF-7 cells were provided by Dr. Carlos Arteaga. Cells were assessed for their viability and counted with a Countess II FL Automated Cell Counter (Life Technologies, Gaithersburg, MD). The cells were regularly tested for Mycoplasma contamination with PlasmoTest (Invivogen).