Abstract Background & Aims : Most gastrointestinal stromal tumors (GISTs) have gain-of-function mutations of c- kit receptor tyrosine kinase (KIT) gene, but some GISTs do not. We investigated the cause of GISTs without KIT mutations. Because GISTs apparently expressed platelet-derived growth factor receptor (PDGFR) α, we examined whether GISTs without KIT mutations had a mutation of PDGFR α. Methods : Whole coding region of PDGFR α complementary DNA (cDNA) was sequenced in GISTs with or without KIT mutations. Mutant PDGFR α cDNA was transfected into 293T human embryonic kidney cells, and autophosphorylation of PDGFR α was examined. Proliferation of Ba/F3 murine lymphoid cells stably transfected with mutant PDGFR α cDNA was estimated by tritium thymidine incorporation. Wild-type KIT cDNA was cotransfected with mutant PDGFR α cDNA, and immunoprecipitation by anti-KIT antibody was performed. Inhibitory effect of Imatinib mesylate on activated PDGFR α was examined. Results : We found 2 types of constitutively activated mutations of PDGFR α, Val-561 to Asp or Asp-842 to Val, in 5 of 8 GISTs without KIT mutations but not in 10 GISTs with KIT mutations. Stable transfection of each mutation induced autonomous proliferation of Ba/F3 cells. Constitutively activated mutant PDGFR α bound and activated the cotransfected wild-type KIT. The constitutive activation of PDGFR α with Val-561 to Asp was inhibited effectively by Imatinib mesylate but that of PDGFR α with Asp-842 to Val was inhibited only weakly, even at the concentration of 10 μmol/L. Conclusions : The gain-of-function mutations of PDGFR α appear to play an important role in development of GISTs without KIT mutations.