Mitochondrial complex IV deficiency nuclear type 17 (MC4DN17) is an autosomal recessive neurometabolic disorder with somewhat variable clinical manifestations and severity. Most affected individuals present in early childhood with motor and gait difficulties after normal early development. These motor abnormalities progress to spastic tetraparesis, sometimes resulting in loss of ambulation. Many patients also show episodic developmental regression: some have impaired cognition and dysarthria, although others have normal speech and cognition. More variable features include seizures and sensorimotor polyneuropathy. The clinical features tend to stabilize over time. Brain imaging shows a cavitating leukodystrophy, and laboratory studies show variably decreased levels and activity of mitochondrial respiratory complex IV in patient tissues (Melchionda et al., 2014). For a discussion of genetic heterogeneity of mitochondrial complex IV (cytochrome c oxidase) deficiency, see 220110.