Inclusion Criteria:

- Age 18 or older

- Histologically confirmed metastatic or unresectable GIST with CD117(+), DOG-1 (+), or
KIT mutation

- ECOG PS 0~2

- Primary mutation at KIT exon 9

- Imatinib treatment for less than 4 weeks from the first dose at 400 mg per day

- No prior use of tyrosine kinase inhibitors ((but, patients who have recurrence 6
months after completion of adjuvant imatinib at a dose of 400 mg per day can be
enrolled in this study)

- At least one evaluable disease by RECIST v1.0

- Resolution of all toxic effects of prior treatments (chemotherapy, surgery, RFA,
radiotherapy, and/or TACE)

- Adequate bone marrow function as defined by platelets ≥ 75 x 109/L and neutrophils ≥
1.5 x 109/L (within 1 week prior to the first dose of imatinib at 400 mg per day)

- Adequate renal function, with serum creatinine < 1.5 x ULN (within 1 week prior to
the first dose of imatinib at 400 mg per day)

- Adequate hepatic function with serum total bilirubin < 1.5 x ULN, alanine
aminotransferase (ALT) or aspartate aminotransferase (AST) < 2.5 x ULN in the absence
of liver metastases, or < 5 x UNL in the presence of liver metastases (within 1 week
prior to the first dose of imatinib at 400 mg per day)

- No other malignant disease apart from non-melanotic skin cancer or carcinoma in situ
of the uterine cervix or any other cancer except where treated with curative intent >
5 years previously without evidence of relapse

- Provision of a signed written informed consent

Exclusion Criteria:

- Severe co-morbid illness and/or active infections

- Pregnant or lactating women

- History of other malignancies except basal cell carcinoma and carcinoma in situ of
uterine cervix

- CNS metastasis

- Clinically significant bleeding in GI tract

- GI obstruction or malabsorption

- Known hypersensitivity to imatinib