L1 syndrome is a group of mild to severe X-linked recessive disorders that share a common genetic basis. The spectrum of L1 syndrome disorders includes X-linked complicated corpus callosum dysgenesis, spastic paraplegia 1, MASA syndrome, and X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS). It is also called L1CAM syndrome (for the disorder's causative gene) and CRASH syndrome, an acronym for its primary clinical features: corpus callosum hypoplasia, retardation (intellectual disability), adducted thumbs, spasticity, and hydrocephalus.