L858R next to exon 19 in-frame deletions are the most frequent EGFR mutations (cumulatively accounting for ~90% of all EGFR mutations), well recognized as biomarkers of targeted therapy of non-small cell lung cancer and other cancers with the use of EGFR-specific tyrosine kinase inhibitors. The gene discussed is EGFR; the disease is non-small cell lung carcinoma.