The proto-oncogene KIT encodes the receptor tyrosine kinase KIT. Gain-of-function mutations in the juxtamembrane domain of KIT have been reported in human gastrointestinal stromal tumors. In a family with multiple gastrointestinal stromal tumors and diffuse hyperplasia of myenteric plexus layer, we have identified another mutation of KIT, a single base mutation, resulting in the substitution of Glu for Lys(642) in the kinase I domain, and studied its biological effect in a cellular system. The mouse homologue of the human KIT mutant was generated by site-directed mutagenesis and stably transfected into the interleukin-3-dependent Ba/F3 murine cell line. The oncogenic potential of the mutated KIT was assessed in vitro by a proliferation assay and in vivo by transplantation into nude mice. Transfected Ba/F3 cells grew autonomously in absence of growth factors and formed tumors in nude mice. Substitution of Glu for Lys(642) is an oncogenic mutation in the tyrosine kinase domain of KIT. As germline heterozygous mutation, it causes a diffuse hyperplasia of myenteric interstitial cells of Cajal during embryonic development and occurrence of multiple gastrointestinal stromal tumors at adulthood.