FXN and hyperinsulinemic hypoglycemia, familial, 4: In addition to the biochemical defects, the histochemical analysis was peculiar, with a quite specific pattern of SDH and COX deficiency; this was suggested to be a pathognomonic finding of a myopathy related to Fe-S cluster.9 However, in patients with mutations in FXN, the picture is different, with mainly COX negative fibres and nearly normal SDH staining.51 Other Fe-S diseases are rare and usually present as neurological disorders with minimal myopathic signs, while little is known about their muscle features.