(a) A PMN in patient no. 1 that appeared after 4 weeks of ATRA treatment. (MPO-stained section, original magnification × 14,900.) (b) Higher magnification of a portion of the cytoplasm in (a). Note that, in contrast to the cells in Fig 5, this cell contains two types of cytoplasmic granules, ie, MPO-positive spherical granules and MPO-negative, elongated or dumbbell-shaped, small granules (arrow), corresponding to the primary and secondary granules of neutrophils, respectively. (MPO-stained section, original magnification × 30,000.)