Activating mutations in the KIT receptor tyrosine kinase cause ligand-independent activation of its tyrosine kinase function which initiates a cascade of intracellular signaling involved in tumorigenesis. Exon 9 KIT defines a distinct subset of gastrointestinal tumors that are often located in the small bowel and have an aggressive clinical behavior. Kit is an example of a cancer target that has been successfully blocked with a new small molecule inhibitor, Gleevec (or Imatinib; STI-571). This drug blocks the receptor tyrosine kinase activity in stromal tumors of the gastrointestinal tract. In these tumors, a mutation in the KIT gene constitutively activates the receptor tyrosine kinase independently of ligand binding.