KIT mutations occur in approximately 85% of patients with gastrointestinal stromal tumors (GIST), while Exon 9 is mutated in approximately 10 ~ 15% of all KIT-mutated GIST. Compared to patients with KIT exon 11 mutations, patients with exon 9 mutations tumors show intermediate sensitivity to imatinib. Median duration of benefit from imatinib is approximately 7~12 months compared to 23 months for patients with exon 11 mutations. Patients with exon 9 mutations are more likely to respond to second line sunitinib than patients with other KIT/PDGFRA mutations.