Matsumoto Y, Sawa K, Fukui M, et al. - Given that ultrasensitive methods could identify low-frequency epidermal growth factor receptor (EGFR) T790 M mutation in EGFR tyrosine kinase inhibitor (TKI)-naïve non-small cell lung cancer (NSCLC), researchers investigated how pretreatment T790 M (preT790 M) influences the efficacy of EGFR-TKIs and resistance. They analyzed two independent cohorts including advanced EGFR-mutated NSCLC patients who were managed with first-line EGFR-TKIs. A derivation cohort named cohort A involving 44 cases with treatment initiated between August 2013 and July 2016, as well as a validation cohort between August 2016 and December 2017 (cohort B, n = 22), was examined. In cohort A and cohort B, the detection rates of preT790 M were 40.9% (18/44) and 45.5% (10/22), respectively, by droplet digital polymerase chain reaction, and none by the Cobas EGFR Mutation Test v2. Significantly shorter progression-free survival on EGFR-TKIs was reported in EGFR-mutated NSCLC with high preT790 M. However, post-TKI T790 M resistance may not be necessarily conferred by preT790 M abundance.