Chronic myeloid leukaemia (CML). Light micrograph of a section through bone marrow in a case of CML. This is aplasia (failure to develop) of the bone marrow. Normal bone marrow would have a greater number of active precursor blood cells (purple). Leukaemia is the excessive production of white blood cells. It is classified by the rate of progression of the disease (acute or chronic) and by the type of cells involved. In this case, it is a slow progression (chronic) involving the myeloid tissue found in bone marrow. Myeloid tissue (red marrow) is where the blood cells are produced. Treatment is with drug therapy and bone marrow transplants. Magnification unknown.