L1CAM and Down syndrome: The dysfunction of L1CAM can lead to “the L1 syndrome,” which is X‐linked, including hydrocephalus with stenosis of the Sylvius aqueduct (HSAS; phenotype MIM number 307000), MASA (mental retardation, aphasia, spastic paraplegia, adducted thumbs) syndrome (phenotype MIM number 303350), complicated hereditary spastic paraplegia type 1 (SPG1, phenotype MIM number 303350), and agenesis of the corpus callosum (phenotype MIM number 308840) (Basel‐Vanagaite et al., 2006).