Acute promyelocytic leukaemia. Light micrograph of blood cells from bone marrow in a case of acute promyelocytic leukaemia. Leukaemia is a cancer where certain blood cells form in excess. There are several acute forms, classified according to the type of cell affected. Acute promyelocytic leukaemia (APL) is a subtype of acute myeloid leukaemia (AML). AML is a cancer affecting the myeloid tissue (bone marrow), and APL specifically affects a type of white blood cell precursor (promyelocytes) that form a type of white blood cell known as granulocytes.