EGFR and lung adenocarcinoma: In-frame deletions of exon 19 of the EGFR gene lead to constitutive activation of EGFR tyrosine kinase activity and render the receptor sensitive to tyrosine kinase inhibitors (TKIs), such as gefitinib, erlotinib, and afatinib, in lung adenocarcinoma [46], [47], [48].