FAB categories are created by mixtures of NPMc⁺leukemic cells of different lineages. Combination of myeloid and erythroid cells is taken as an example of how FAB categories are created in NPMc⁺ AML. (Left panel, M1) NPMc⁺ AML of M1-type showing numerous glycophorin A-negative myeloid blasts (blue) and rare glycophorin A-positive (red) proerythroblasts (GLYC, arrow; APAAP; hematoxylin counterstain, ×1000) that express cytoplasmic NPM, as shown by double staining for glycophorin A(brown)/NPM (blue) (G/NPM, arrow; immunoperoxidase/fast blue APAAP, no counterstain, ×1000), and labeling in red (cytoplasmic-restricted) of the mutant NPM (Sil-A, arrow; APAAP; hematoxylin counterstain, ×1000). (Middle panel, M6a) NPMc⁺ AML of M6a type characterized by equal representation of myeloperoxidase-positive myeloid blasts (MPO, red; APAAP; hematoxylin counterstain, ×400) and glycophorin A-positive erythroid cells (GLYC, red; APAAP; hematoxylin counterstain, ×1000). Erythroid cells double stain for surface glycophorin A (brown)/cytoplasmic NPM (blue) (G/NPM, arrow) and for surface glycophorin A (brown)/nuclear C23 (blue) (G/C23, arrow; immunoperoxidase/fast blue APAAP, no counterstain, ×1000). (Right panel, M6b) NPMc⁺ AML of M6b type showing a predominant population of erythroid precursors (GLYC, red; APAAP; hematoxylin counterstain, ×1000) expressing cytoplasmic NPM as documented by double staining for surface glycophorin A (brown)/cytoplasmic NPM (blue) (G/NPM; immunoperoxidase/fast blue APAAP, no counterstain, ×1000), and by labeling in red (cytoplasmic-restricted) for mutant NPM (Sil-A, arrow; APAAP; hematoxylin counterstain, ×1000); T indicates a bone trabecula.