COQ9 and mitochondrial complex IV deficiency, nuclear-type: Moreover, the development of rational therapies must provide better therapeutic outcomes with minimum side-effects, as it has been demonstrated, at preclinical level, with the dNTPs/nucleosides therapy in Tk2 deficiency [40,41], with AICAR administration or OPA1 overexpression in COX deficiency [20,42], or with the β-resorcylic acid therapy in cases of CoQ deficiency due to defects in the CoQ biosynthetic genes Coq7 or Coq9 [9,43].