EGFR and neoplasm: Indeed, Yasuda et al. also investigated another exon 20 insertion that occurred within the C-helix of EGFR, A763_Y764insFQEA, and unexpectedly found that this mutation had a high affinity for gefitinib in vitro and was highly sensitive to erlotinib in an engineered cell line model.22 Three patients with the A763_Y764insFQEA insertion showed tumor regression or stable disease following erlotinib treatment, leading the authors to conclude that unlike other mutations of its class, this particular exon 20 insertion is sensitive to first-generation EGFR inhibitors.