Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the gastrointestinal tract. Most GISTs characteristically express the KIT receptor tyrosine kinase, and approximately 80% harbor oncogenic mutations within the KIT gene. These primarily involve KIT exon 11, but may also occur in exons 9, 13, and 17. All mutations result in the constitutive activation of KIT, which has led to the use of tyrosine kinase inhibitor drugs (imatinib) as a common therapy for GIST patients. Testing for KIT mutation can aid in the diagnosis of GIST, particularly for tumors that lack KIT expression by immunohistochemistry. Mutation testing may also aid in determining prognosis, and may help predict response to imatinib therapy, both of which have been associated with specific KIT mutations. This DNA sequencing test will detect mutations within exons 9, 11, 13, and 17 of the KIT gene. The tested exons correspond to KIT amino acids 497-513 (exon 9), 550-591 (exon 11), 628-661 (exon 13), and 799-823 (exon 17). Appropriate specimens include formalin fixed paraffin embedded tissue blocks and fresh/frozen tissues. Specimens should contain an adequate proportion of tumor nuclei (>40%) to enable mutation detection.