 - Post-transplant treatment: Patients experiencing disease persistence or progression AND low donor chimerism discontinue immunosuppression. Patients with disease persistence or progression after discontinuing immunosuppression receive oral imatinib mesylate once daily. Patients who have disease improvement after day 28 of imatinib mesylate treatment AND who have no evidence of disease after day 84 of imatinib mesylate treatment continue imatinib mesylate in the absence of disease progression or unacceptable toxicity. Patients who fail to improve after day 28 of imatinib mesylate treatment OR who have residual disease after day 84 of imatinib mesylate treatment receive donor lymphocytes IV over 15-30 minutes once every 1-4 months for up to 4 infusions. Patients ineligible to receive donor lymphocytes e.g., patients with evidence of GVHD receive interferon alfa subcutaneously 3 times a week for up to 12 months in the absence of disease progression or unacceptable toxicity. 