Autosomal recessive spastic paraplegia type 21 is a complex type of hereditary spastic paraplegia characterized by an onset in adolescence or adulthood of slowly progressive spastic paraparesis associated with the additional manifestations of apraxia, cognitive and speech decline (leading to dementia and akinetic mutism in some cases), personality disturbances and extrapyramidal (e.g. oromandibular dyskinesia, rigidity) and cerebellar (i.e. dysdiadochokinesia and incoordination) signs. Subtle abnormalities (e.g. developmental delays) may be noted earlier in childhood. A thin corpus callosum and white matter abnormalities are equally reported on magnetic resonance imaging. The disease is Monarch Disease Ontology entry MONDO_0009568 (mast syndrome). Also known as: Mast syndrome, SPG21, autosomal recessive spastic paraplegia 21, autosomal recessive spastic paraplegia type 21, hereditary spastic paraplegia 21.