(a) A PMN in patient no. 1 that appeared in the bone marrow after 3 weeks of ATRA treatment. There are numerous granules in the cytoplasm, including spherical, elongated, and even dumbell-shaped granules, all of which are MPO-positive, ie, primary granules. A lipid droplet (L) is also present. (MPO-stained section, original magnification × 15,200.) (b) Higher magnification of a portion of the cytoplasm in (a). MPO-negative secondary granules are not seen. (MPO-stained section, original magnification × 30,900.)