EGFR and neoplasm: Moreover, the frequency of EGFR T790M mutation was greatly lower in TKI‐relapsed samples with <20% tumor cellularity than in samples with ≥20% tumor cellularity, suggesting that 20% tumor purity should be the minimum requirement to identify T790M mutation as the cause of TKI resistance in rebiopsy samples using the amplification‐based NGS testing.