Chronic myeloid leukaemia. Light micrograph of blood cells from bone marrow in a case of chronic myeloid leukaemia. Leukaemia is a cancer where certain blood cells form in excess. There are several chronic forms, classified according to the type of cell affected. Chronic myeloid leukaemia is a cancer affecting the myeloid tissue (bone marrow), specifically the white blood cell precursors (myeloblasts) that form a type of white blood cell known as granulocytes. The pink cells are precursor or white blood cells. The pale orange ones are red bloods cells. Magnification: x1000 when printed at 10 centimetres across.