SCO1 and mitochondrial complex IV deficiency, nuclear type 4: Mitochondrial complex IV deficiency nuclear type 4 (MC4DN4, OMIM #619048) is an autosomal recessive disorder caused by homozygous or compound heterozygous pathogenic variants in the SCO1 gene (chromosome 17p13.1), which encodes for an assembly factor of COX (3) Associated phenotypes include severe encephalopathy, hepatopathy, lactate acidosis, and hypertrophic cardiomyopathy.