Interestingly, the clinical correlation between the pres- ence of specific mutations and therapeutic response to TKIs is mirrored in cell lines and EGFR-transfected cells. Cells bearing the mutant EGFR are in general more sensi- tive to TKIs than cells expressing the wild-type kinase. In particular, the L858R mutant is 10- to 100-fold more sen- sitive to erlotinib and gefitinib than the wild-type kinase (Pao et al., 2004; Greulich et al., 2005; Mukohara et al.,
2005) and significantly more sensitive than the G719S mutant (Jiang et al., 2005). At the same time, the excep- tions to this rule—for example, the exon 20 insertion mutants are highly resistant to both gefitinib and erlotinib (Greulich et al., 2005)—further underscore the depen- dence of inhibitor responses on specific mutations.