EGFR and esophageal squamous cell carcinoma: Note that, unlike gefitinib-resistant ESCC cell lines derived previously, which were generated by exposure to increasing incremental doses of EGFR TKi [13], our panel of resistant lines were generated by the addition of just above (2 μM gefitinib) peak plasma levels (1–1.4 μM) of gefitinib from the outset.