Hematoxylin & eosin (A, B), modified Gomori trichrome (C, D), Nicotinamide adenine dinucleotide–tetrazolium reductase (E, F), cytochrome c oxidase (COX) (G, H), serial, and ATPase pH4.5 (I, J) staining of the biopsied muscle samples from the biceps brachii of patient III-2 in family 1 (left panels, A, C, E, G, I) and the triceps brachii of patient IV-1 in family 2 (right panels, B, D, F, H, J) are shown. Hypertrophic fibers larger than 100 μm and angulated or rounded atrophic fibers are shown (A, B). Highly increased central nuclei and a fraction of fibers with pyknotic nuclear clumps are present (B). Rimmed vacuoles are located in atrophic fibers, which tend to make small groups (C, D). Disorganization of the myofibrillar network is observed (F). COX staining does not display complete COX-deficient myofibers (G, H). On ATPase pH4.5, the type distribution is almost equal, and atrophic fibers are observed in both type 1 and type 2 fibers (I, J). Type grouping is essentially negative except for the subtle finding of several type 1 fibers making a small group (I, J). Scale bars = 100 μm.