Four phenotypic groups can be defined: (1) SRNS, isolated or with neurological involvement, associated with defects in PDSS2, COQ2, COQ6, or COQ8B (the latter with later age-at-onset); (2) encephalomyopathy, hypertrophic/dilated cardiomyopathy, lactic acidosis, and tubulopathy with defects in PDSS2, COQ2, COQ7, or COQ9; (3) neonatal cardio-encephalopathies with COQ2, COQ4, or PDSS1; and (4) pure neurological syndromes, including isolated or combined Leigh syndrome, ARCA, and refractory epilepsy, in association with defects in COQ2, COQ4, COQ5, COQ7, or COQ9 [Table 1 and Figure 2][9,10]. The gene discussed is COQ4; the disease is mitochondrial DNA depletion syndrome, encephalomyopathic form with methylmalonic aciduria.