Mm134

The ER⁺ lobular breast cancer cell line MDA-MB-134 (MM134) was purchased from the ATCC. MM134 cells were cultured in DMEM:Leibovitz-15 1:1 media supplemented with 10% FBS and 1% penicillin/streptomycin. MCF7 breast cancer cell line (HER2 WT or the L755S HER2-activating mutation) were previously isogenically modified using AAV-mediated gene targeting as previously described (16 (link)). These MCF7 breast cancer cells were gifts from Dr. Ben Ho Park (Vanderbilt University Medical Center, Nashville, TN). These cells were cultured in IMEM supplemented with 10% FBS and 1% penicillin/streptomycin. SUM44PE cells were purchased from BIOIVT. SUM44PE parental or CRISPR knock in (KI) V777L and L755S cells were cultured in Ham's F12 media with 2% FBS and 1% penicillin/streptomycin supplemented with all the additional components listed in Supplementary Table S2 as per BIOIVT recommendations. For low E2 growth conditions, the appropriate phenol-red-free medium supplemented with 5% charcoal-dextran-stripped serum was used. All cell lines were cultured in 5% CO₂ at 37°C and were examined every 6 months for Mycoplasma.