ESR1 and breast cancer: The gene abnormalities covered by our BC-monitor are important in practice; many of them indicate a poor prognosis (MET, CDH1, TP53, PIK3CA, ESR1, and HER2); the presence of resistance mechanisms (PIK3CA, ESR1, HER2, and FGFR1); or represent druggable targets (PIK3CA, ESR1, HER2, and AKT1) at the same time.