ESR1 and metastatic neoplasm: In a prospective analysis of 155 plasma samples derived from patients with advanced BC, with plasma samples collected at any time during metastatic disease, ESR1 mutations detected by ddPCR were found to be less frequent in BC treated with CDK4/6 inhibitors in combination with fulvestrant (113 of 155 patients had ER+/ HER2-negative disease; 34 of these harbored an ESR1 mutation, two patients treated with combination palbociclib and fulvestrant = 5.9%), than in patients treated with fulvestrant alone (32 treated with fulvestrant alone = 94.1%; p = 0.01) [39].