Somatic mutations in the epidermal growth factor receptor (EGFR) correlate with increased response in patients with non-small-cell lung cancer (NSCLC) treated with EGFR tyrosine kinase inhibitors (TKIs). The multicenter iTARGET trial prospectively examined first-line gefitinib in advanced NSCLC patients harboring EGFR mutations and explored the significance of EGFR mutation subtypes and TKI resistance mechanisms.