In MOCD, the clinical picture of sulfite oxidase deficiency, an enzyme involved in the metabolism of sulphites, dominates that of xanthine dehydrogenase [9], though elevated biochemical markers are due to deficiency of both xanthine dehydrogenase (xanthine and hypoxanthine) and sulfite oxidase (S-sulphocysteine) (Fig. 1). The gene discussed is SUOX; the disease is isolated sulfite oxidase deficiency.