PET100 and Leigh syndrome: Previous studies in yeast identified PET100 gene as a COX biogenesis factor,21, 22, 23 and more recently a Lebanese PET100 founder mutation has been described in 10 individuals presenting with Leigh syndrome.24 Fibroblasts and skeletal muscle of our patient showed impaired complex IV activity, associated with a profound defect in COX assembly, and decreased steady-state levels of complex IV proteins.