Figure 1 shows what normal white blood cell production is like. In a person with leukemia, the bone marrow produces abnormal white blood cells that are called leukemia cells and leukemic blast cells. The abnormal cells can’t produce normal white blood cells. Leukemia cells divide to produce copies of themselves. The copies divide again and again, producing more and more leukemia cells. Unlike normal blood cells, leukemia cells don’t die when they become old or damaged. Because they don’t die, leukemia cells can build up and crowd out normal blood cells. The low level of normal blood cells can make it harder for the body to get oxygen to the tissues, control bleeding, or fight infections. Also, leukemia cells can spread to other organs, such as the lymph nodes, spleen, and brain. In CML patients, abnormal white blood cell production is caused by the Bcr-Abl, a protein formed by the Philadelphia chromosome. The Philadelphia chromosome is a chromosomal defect found in 90% of CML patients. Its formation in shown in Figure 2. The BCR-Abl pathway can be interrupted by Gleevec (imatinib mesylate) as shown in Figures 3 and 4. Mouse over the images to find more information.