TK2 and proximal spinal muscular atrophy: The genes known to cause SMA phenotypes with mitochondrial disorders are SCO2, TK2, and DGUOK. SCO2 encodes one of the COX assembly proteins associated with copper insertion into the holoenzyme involved in the biogenesis of COX subunit II that is important in the function of the mitochondrial inner membrane while the latter two are mitochondrial depletion syndromes, characterised by reductions in the amount of mitochondrial DNA which impairs the synthesis of respiratory chain complexes.