PM-COX staining patterns. PM-COX cases were histologically similar to classic PM, with endomysial inflammation, fiber invasion, and lack of well-developed chronic myopathic features (a and f; H&E, frozen material), but with ≥ 1% COX-negative fibers (b and g; COX stain, frozen material; COX-negative fibers are marked by asterisks). The majority of PM-COX samples (designated PM-COX low) showed no significant sarcoplasmic labeling for LC3 (c), p62 (d), or TDP-43 (e). In a subset of samples (designated PM-COX high), LC3 labeled RV rims (h), p62 labeled RV rims and small protein aggregates (i), while TDP-43 labeled sarcoplasmic skeins and large protein aggregates (arrows; j). a-e, subject #37, f-j, subject #33; scale bar, 50 μM.