ESR1 and breast cancer: M/D-driven tumors emerge in immunocompetent hosts via an ER-dependent oncogenic process, and progress as they evade immunosurveillance, culminating with the establishment of mammary tumors with a luminal B (highly proliferative HER2−ER+VIM−) phenotype that fails to convert into basal (ER−VIM+) over time or upon treatment.