FAB M0. Marrow blasts from patients with this undifferentiated type of acute myelogenous leukemia can have variable amounts of agranular cytoplasms with Auer rods. Cells are peroxidase- and Sudan black-negative and can be confused with FAB M7 or FAB L2. Myeloid commitment of these blasts can be confirmed by immunophenotyping with antibodies against myeloid antigens and/or demonstration of ultrastructural peroxidase-positive granules using transmission electron microscopy.