Professor Tsuneyoshi group evaluated in detail the fact that gastrointestinal stromal tumor occur in the soft tissue of post-peritoneal space and peritoneum. These tumors show the same mutations of c-kit gene as gastrointestinal stromal tumor. Then, they prospect the therapy using Imatinib, molecular target drug for gastrointestinal stromal tumor, may be effective for these extra-gastro interstitial soft tissue tumors.