Pathology characteristic of HIBM in biceps muscle biopsy from proband. A) Hematoxylin and eosin staining showed marked variation in fiber diameter, rounded fibers, mild endomysial fibrosis, slight fatty infiltration, and irregular vacuoles (arrows) containing granular material. Frankly necrotic fibers, basophilic regenerating fibers, and inflammatory infiltrates were not prominent. The disease process appeared focal, with fibers in some areas mildly affected, while other regions, including the one shown here, exhibited severe myopathic pathology. B) Modified Gomori trichrome staining highlighted small amounts of granular red material within and around the vacuoles (arrow), as well as endomysial fibrosis and small, angulated, atrophic fibers.