EGFR exon 20 insertion testing identifies a distinct subset of lung adenocarcinomas, accounting for at least 9% of all EGFR-mutated cases and by molecular modeling, are predicted to have potentially different effects on erlotinib binding. Studies show that in contrast to the more classic activating EGFR mutations, these insertions have been associated with de novo resistance to approved EGFR-TKIs (erlotinib and gefitinib). In a recent study, patients with advanced lung adenocarcinoma harboring exon 20 insertions demonstrated no response or partial response following treatment with TK inhibitors. This rare complex mutation (p.H773_V774delinsLM) results in the H773L/V774 mutation compound at the same allele, potentially weakening the inactive state and leading to constitutional activation of EGFR. A recent clinical report suggests this mutation is insensitive to the reversible TKI gefitinib, but can be suppressed by the irreversible TKI osimertinib, leading to sustained disease control (Yang et al., Lung Cancer, 121:1-4, 2018). Exon 20 insertion mutations in EGFR may be associated with clinical trials (https://clinicaltrials.gov/).