Nevertheless, complex IV defects are common secondary effects of an impairment of Fe-S protein biogenesis.6 49 A complex IV deficiency (19%–28% residual activity) was also reported in the two siblings with the most severe ISCU-related phenotype, and a partial reduction was found in five subjects with the Swedish-type myopathy.14 This may be due to a downstream damaging effect linked to impairment of complexes I, II and III or to the hampered formation of MRC supercomplexes. Here, CD200 is linked to myopathy.