That notwithstanding, since a small percentage of patients with GIST do not express detectable levels of KIT or do not harbor mutations within KIT/PDGFR (44–46) IHC or mutational analysis should not be used to deny treatment with imatinib mesylate since these patients may still be sensitive to this therapy. The gene discussed is PDGFRB; the disease is gastrointestinal stromal tumor.