FAD and FMN are cofactors derived from the metabolism of riboflavin, which has been employed in the treatment of several disorders involving different enzymatic complexes of the OXPHOS system: complex I deficiency [36], [42], [43], complex II deficient patients, at least by reducing the rate of disease progression [40], and in a boy with Leigh syndrome and complex II deficiency [44]. This evidence concerns the gene FMN1 and Leigh syndrome.