Patients will be randomly assigned to an imatinib arm with either intermittent or continuous dosing schedule with a ratio of 1:1 by using a computer-based system. Imatinib will be administered at a dose of 400 mg/day, once a day with food, in the form of 100-mg tablets. Patients assigned to the continuous dosing arm will receive imatinib without off-schedule, and those assigned to the intermittent dosing arm will received imatinib with one-week on/one-week off dosing schedule. Four weeks of study treatment is considered as one cycle for both continuous and intermittent dosing schedules.