Though patients whose tumors harbor EGFR T790M mutation appear to benefit from rociletinib, there is a need to understand the molecular mechanisms that lead to primary and acquired resistance to rociletinib. The investigators propose to conduct a clinical trial of rociletinib of patients with EGFR-mutant NSCLC with activating EGFR mutations (including exon 19 deletion or L858R mutation), with or without EGFR T790M mutation. In these patients, pre-treatment and post-progression biopsy specimens will be subjected to genomic analysis to fully understand the clonal evolution and the molecular mechanisms underpinning treatment resistance.