ESR1 and breast cancer: The ER+ BC cell line, MCF7, faithfully recapitulates ER+ BC development from in situ to metastatic disease when grafted this way (Sflomos et al, 2016) although the cell line is derived from a pleural effusion, i.e., late‐stage disease, and has been passaged extensively in vitro (Soule et al, 1973).