DM muscle features: (a) 1 and 2, or (b) 1 and 3, or (c) 2 and 3, or (d) 4.1. Proximal muscle weakness.2. Elevated muscle enzymes.3. Suggestive DM muscle biopsy findings: lymphocytic infiltrate (perivascular), evidence of perifascicular disease (perifascicular predominant fibers that are pale on COX staining and/or positive on NCAM staining).4. Definitive DM muscle biopsy findings: perifascicular atrophy and/or perifascicular MxA overexpression with rare or absent perifascicular necrosis. Here, MX1 is linked to glycogen storage disease VI.