For this reason, this study is designed to examine ABL point mutations in patients with Philadelphia chromosome-positive or BCR-ABL-positive chronic leukemia who are treated with imatinib, to observe dynamics of existing point mutations during treatment with nilotinib, and to confirm the mechanism of resistance including expression of new point mutations and their expression patterns through a long-term follow-up. In addition, this study will confirm whether expression pattern of ABL point mutations associated with nilotinib in Asian is different from that in Western patients.