ESR1 and breast cancer: Analysis of ctDNA can be used in metastatic breast cancer (BC) to identify important targets, such as mutations in the catalytic subunit alpha of phosphatidylinositol-4,5-bisphosphate 3-kinase (PIK3CA) or the estrogen receptor 1 (ESR1) gene, predictive of response to alpelisib5,6, or elacestrant7,8, respectively.