Marrow is the soft, fatty tissue inside the bones.
The cancer can cause the number of white blood cells in the bone marrow to be 20 to 40 times higher than normal. These cancerous white blood cells crowd out healthy blood cells in the bone marrow. The cause of this cancer is not known. However, often it appears to be related to the presence of an abnormal chromosome called the Philadelphia chromosome. CML develops mainly in people who are middle-aged and sometimes in people who are older.