ESR1 and breast cancer: Baseline plasma cell‐free DNA (cfDNA) from 164 postmenopausal women with ER‐positive, HER2‐negative MBC refractory to a nonsteroidal aromatase inhibitor and treated with standard EVE/EXE (Everolimus Biomarker Study, Eudract 2013‐004120‐11) was characterised for 10 relevant breast cancer genes by next‐generation sequencing with molecular barcoding.