Cerbelli B, et al. - Herein, the authors explored the anti-aminoacyl-tRNA synthetase-related myositis and dermatomyositis for the differential diagnosis on muscle biopsy. Decreased cytochrome c oxidase (COX) stain and mitochondrial DNA depletion were revealed, consistent with mitochondrial dysfunction. Amino acyl-tRNA synthetases (ARS) exhibited scattered COX-deficient fibers, not restricted to the perifascicular region, in contrast to DM. The occurrence of this feature was noticed in up to 91% of patients, being prominent only in two.