Somatic mutations in the tyrosine kinase domain of the epidermal growth factor receptor (EGFR) gene are present in approximately 80% of the lung adenocarcinomas that respond to first and second generation EGFR inhibitors (eg, gefitinib, erlotinib and afatinib). Two types of mutations account for approximately 80-90% of all EGFR mutations: short in-frame deletions in Exon 19 and a point mutation in exon 21 at codon 858 (L858R). Other less common mutations in exons 18, 20, and 21 are found in 10-20% of EGFR-mutated cases. EGFR exon19 deletions, exon 21 L858R and Exon 18 mutations correlate strongly with sensitivity to specific EGFR inhibitors, and the response rate to therapy with TKIs has been reported to be up to 80% in such cases. The T790M mutation in exon 20 is associated with resistance to some EGFR inhibitors. However, third generation TKI (eg, osimertinib) can specifically target T790M. Compound (dual) mutations in EGFR have been previously reported in lung adenocarcinoma and typically include a strong activating mutation combined with a weaker activating mutation. These cases appear to respond well to the EGFR targeted therapies. Mutations at E709 in exon 18 often occur together with other mutations in EGFR. This particular complex deletion insertion variant results in both the E709V and G719C in the protein, as well as a K713R variant, which also has been reported previously.