L1 syndrome (MONDO_0017140, a Monarch Disease Ontology id) is a mild to severe congenital X-linked developmental disorder characterized by hydrocephalus of varying degrees of severity, intellectual deficit, spasticity of the legs, and adducted thumbs. The syndrome represents a spectrum of disorders including: X-linked hydrocephalus with stenosis of the aqueduct of Sylvius (HSAS), MASA syndrome, X-linked complicated hereditary spastic paraplegia type 1, and X-linked complicated corpus callosum agenesis. Also known as: CRASH syndrome, L1CAM syndrome, corpus callosum hypoplasia-retardation-adducted thumbs-spasticity-hydrocephalus syndrome.