CD200 and inborn mitochondrial metabolism disorder: For the diagnosis of mitochondrial diseases, skeletal muscle is often considered the tissue of choice (Thorburn and Rahman 1993), and fibroblasts have been considered less sensitive than skeletal muscle biopsy samples, detecting MRC defects in only half of cases with positive skeletal muscle assay results (Thorburn et al. 2004; Heuvel et al. 2004).