Illustration of blood cells in chronic myeloid leukaemia (CML). This cancer affects the myeloid tissue (bone marrow), specifically the white blood cell precursors (myeloblasts) that form a type of white blood cell known as granulocytes. These immature cells (dark purple) fill up the bone marrow and spill into the bloodstream and circulate throughout the body. The cells do not work properly to fight infections and their increased production reduces the production of red blood cells leading to anaemia and fatigue. Chronic leukaemias progress slowly and may not be diagnosed for a number of years.