Caption:
A drawing of blood cells with leukemia, showing platelets (blue), red blood cells (red), white blood cells (light purple) and blast cells (orange). Leukemia is a group of cancers that usually begin in the bone marrow and result in high numbers of abnormal white blood cells. The exact cause of leukemia is unknown. Different kinds of leukemia are believed to have different causes. Both inherited and environmental (non-inherited) factors are believed to be involved. Risk factors include smoking, ionizing radiation, some chemicals (such as benzene), prior chemotherapy, and Down syndrome. People with a family history of leukemia are also at higher risk.