Patients with CML (chronic myelogenous leukemia) who have received constant treatment with Imatinib (Gleevec) have acquired point mutations in the tyrosine kinase domain of the BCR-ABL fusion gene. Especially, the T315I mutation is known to be a mutation associated with imatinib resistance and is observed with high frequency. It is important to monitor patients treated with the Imatinibby BCR-ABLfusion gene mutation test to prevent failure of therapy due to mutational drug resistance.