['Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the human gut. They frequently have gain-of-function mutations of the c-kit gene, which encodes a receptor, tyrosine kinase. The mutations were found at exon 11 in most cases, and either at exon 9 or at exon 13 in rare cases. Recently, we found a family with multiple GIST and a gain-of-function mutation at exon 17. The family was the first reported GIST case with c-kit gene mutation at exon 17 including sporadic GIST. Although we previously reported that the c-kit gene mutation at exon 17 was not detected in 124 sporadic GIST by single-strand conformation polymorphism (SSCP) analysis, the mutation at exon 17 observed in the familial GIST was detectable by the use of direct sequencing but not by our SSCP method. In the present study, we examined the mutations at exon 17 and exon 13 by using direct sequencing.', 'Genomic DNA was extracted from formalin-fixed, paraffin-embedded GIST tissues. We could obtain 143 sporadic GIST cases appropriate for DNA analysis at exon 17 and 141 at exon 13. Exons 17 and 13 were amplified by using polymerase chain reaction and direct sequencing was conducted.', 'No mutation was found at exon 17, and only one case with the mutation at exon 13 was observed. The GIST with the mutation at exon 13 was large and showed frequent mitosis, and the patient died of the recurrent GIST 3 years after the first operation.'] Is c-kit gene mutation at exon 17 or 13 very rare in sporadic gastrointestinal stromal tumors?