Patients with advanced non-small-cell lung cancer and who do not carry an activating somatic EGFR mutation, for example the TT genotype at rs121434568 (also known as L858R), may have a decreased response to gefitinib, as measured by response rate and progression-free survival time, as compared to patients who have an activating EGFR mutation, for example the GG or GT genotypes at rs121434568. Many of the studies listed here combine patients with different activating somatic EGFR mutations, such as L858R and exon 19 deletions, together for analysis. Other genetic and clinical factors may also affect response to gefitinib.