The neural cell adhesion molecule L1 (NCAM-L1) is an immunoglobulin superfamily adhesion molecule that is critical for neural development, involved in neuron-neuron adhesion, neurite fasciculation, outgrowth of neurites, etc. Defects in L1CAM are the cause of hydrocephalus due to stenosis of the aqueduct of Sylvius (HSAS). Hydrocephalus is a condition in which abnormal accumulation of cerebrospinal fluid in the brain causes increased intracranial pressure inside the skull. Defects in L1CAM are also the cause of mental retardation, aphasia, shuffling gait, and adducted thumbs syndrome (MASA), a X-linked disorder.