Chronic granulocytic leukemia (CML) is a malignancy of the hematopoietic system that usually has a slow progression. Under normal circumstances, the bone marrow produces normal blood cells, whereas in patients with CML, the bone marrow produces large numbers of abnormal white blood cells. A defective chromosome, the Philadelphia chromosome, known as the Ph chromosome, is found in almost all patients with CML (about 95 percent) and is so named because it was first discovered in Philadelphia, USA. Sun Hui, Department of Hematology, First Affiliated Hospital of Zhengzhou University Every cell in the human body, except for red blood cells, contains chromosomes. Chromosomes consist of DNA that contains specific information about an individual and are located in the nucleus in the center of the cell. Each chromosome is named in order of its size, with the largest being chromosome 1 and the smallest being chromosome 22. There is also one pair of sex chromosomes, also known as the X and Y chromosomes. The sex chromosomes in males consist of one X chromosome and one Y chromosome, while females consist of two X chromosomes. The Ph chromosome is a shorter than normal chromosome 22 due to an exchange of genetic material between chromosome 9 and chromosome 22, known as a t(9;22) translocation. The translocation generates a new defective fusion gene, bcr-abl, which produces the fusion protein P-210, and in a small number of patients, P-190. The fusion protein acts as a messenger that sends the wrong instructions to the bone marrow, causing it to produce a large number of CML leukemia cells. If diagnosed with CML, the treatment of choice is imatinib mesylate, or Gleevec, which has proven efficacy and safety and is significantly superior to allogeneic hematopoietic stem cell transplantation. Imatinib, the first synthetic small molecule targeted therapy drug in the history of oncology treatment, is a milestone in the treatment of CML and exerts targeted therapeutic effects by blocking signals sent from the Philadelphia chromosome to the bone marrow to restore diseased cells in the blood to normal.