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A neurodevelopmental disorder manifesting with a wide range of clinical symptoms. Clinical features range from severe motor and cognitive impairment with marked choreoathetosis, self-injurious behavior and epileptic encephalopathy, to a milder phenotype featuring moderate developmental delay associated with complex stereotypies, mainly facial dyskinesia, and mild epilepsy. Hyperkinetic movements are often exacerbated by specific triggers, such as illness, emotion and high ambient temperature. Some patients have brain abnormalities, such as cerebral atrophy or thin corpus callosum.