Chronic lymphocytic leukaemia. Light micrograph of blood cells from bone marrow in a case of chronic lymphocytic leukaemia, also called chronic lymphoid leukaemia (CLL). Leukaemia is a cancer where certain blood cells form in excess. There are several forms, classified according to the type of cell affected. CLL is a cancer where excess white blood cells called lymphocytes form. This sample shows diffuse infiltration by mature lymphocytes (dark red).