These related neurological syndromes with X-linked inheritance are allelic disorders due to mutation in the L1CAM gene. Congenital hydrocephalus and resultant macrocephaly due to stenosis of the aqueduct of Sylvius may occur as an isolated finding, but is frequently associated with other features, including hypoplastic or flexed, adducted thumbs. Patients exhibit varying degrees of mentally retardation and spastic paraplegia, particularly of the lower extremities. MASA syndrome includes Mental retardation, Aphasia, Shuffling gait, and Adducted thumbs. In addition, CRASH syndrome includes Corpus callosum agenesis/hypoplasia, Retardation, Adducted thumbs, Spastic paraplegia, and Hydrocephalus. There can be significant phenotypic variability within families, with some males severely affected and diagnosed prenatally while others may have no macrocephaly and long survival. Approximately 5% of females harboring a L1CAM mutation exhibit clinical disease symptoms.