Twenty-nine patients with advanced stage NSCLC and EGFR driver mutations detected by standard techniques were enrolled…. Median progression-free survival was shorter in patients whose tumors having EGFR copy number gain (7.0 vs 23.0 months, p = 0.022)....HCC827 with decreased EGFR copy number demonstrated growth inhibition with 0.1 nM afatinib. The absence of EGFR copy number gain detected in ctDNA may be predictive markers of long-term response to afatinib.