Clonal evolution, the accumulation of genetic and epigenetic changes over time in individual cells,117, 118 is now believed to be the root of drug resistance.119, 120 Biopsies that were taken after the failure of rociletinib targeting EGFR mutant (T790M)-expressing lung cancer121 showed that at least a portion of the resistant tumor still expressed the T790-wild-type protein.120 The wild-type clones existed before treatment with rociletinib. The gene discussed is EGFR; the disease is neoplasm.