MMDS3 is an autosomal recessive severe neurodegenerative disorder characterized by loss of previously acquired developmental milestones in the first months or years of life. Some affected patients have normal development in early infancy before the onset of symptoms, whereas others show delays from birth. Features included loss of motor function, spasticity, pyramidal signs, loss of speech, and cognitive impairment. The disease course is highly variable: some patients die of respiratory failure early in childhood, whereas some survive but may be bedridden with a feeding tube. Less commonly, some patients may survive and have a stable course with motor deficits and mild or even absent cognitive impairment, although there may be fluctuating symptoms, often in response to infection. Other variable features include visual problems and seizures. Brain imaging shows diffuse leukodystrophy in the subcortical region, brainstem, cerebellum, and spinal cord. Laboratory studies tend to show increased lactate and CSF glycine, and decreased activity of mitochondrial complexes I and II, although these findings are also variable. There may be additional biochemical evidence of mitochondrial dysfunction. [From OMIM]