COX4I1 and Pelizeaus-Merzbacher spectrum disorder: The Leigh syndrome spectrum is a genetically heterogeneous PMD, caused by the dysfunction of one of 113 genes [18], including some related to COX: MT-CO1, MT-CO2, MT-CO3, COX4I1, COX8A, COX10, COX15, LRPPRC, PET100, PET117, SCO2, SURF1, TACO1.