(a) A PMN in patient no. 1, seen in the bone marrow after 3 weeks of ATRA treatment. Lobulated nuclei are connected by nuclear filamentous structures and heterochromatin is highly condensed along the nuclear membrane. An Auer rod (A) is present along with primary granules. (Original magnification × 16,900.) (b) Higher magnification of the cytoplasm of a PMN in patient no. 3 after 3 weeks of ATRA treatment. Auer rods and cytoplasmic granules are all positive for MPO, and no MPO-negative secondary granules are evident. (MPO-stained section, original magnification × 41,900.)