(a) A PMN that emerged in the APL cell culture of patient no. 3 in the presence of ATRA for 1 week. From the lobulated nuclear configurations and condensed heterochromatin, this cell looks like a PMN. However, the cytoplasm contains many MPO-positive primary granules but no secondary granules. (MPO-stained section, original magnification × 14,900.) (b) Higher magnification of a portion of the cell in (a), showing that the cytoplasmic granules are all MPO-positive. (MPO-stained section, original magnification × 30,100.)