EGFR tyrosine kinase inhibitor that binds to the ATP-binding site of the enzyme. The functions of the EGFR tyrosine kinase in activating the Ras signal transduction cascade and malignant cell growth are thus inhibited. It has been shown that a mutation in the EGFR tyrosine kinase domain is responsible for activating anti-apoptotic pathways in gefitinib-sensitive non-small cell lung cancers. These mutations tend to increase sensitivity to tyrosine kinase inhibitors including gefitinib and erlotinib. The adenocarcinoma subset of non-small cell lung cancer histologies often have these mutations, which are commonly found in Asians, women, and non-smokers.