second and sixth in the ranking of missense variants. We did not find any missense variants in the complete coding region of CASK in 390 control X chromosomes. Mental retardation was mild to moderate in the four families with missense variants. In two, it was accompanied by nystagmus, a highly unusual accessory feature of XLMR, in multiple affected individuals. Three of the four variants segregate completely with mental retardation (Fig. 1c). The fourth variant, in family 74, is present in the three individuals with both mental retardation and nystagmus, but is absent from an individual with mental retardation without nystagmus (III-4), who may be a phenocopy. While this manuscript was under review, heterozygous inactivating mutations of CASK were reported to cause severe cerebral malformation in females22 and, in a male, a hemizygous truncation caused early neonatal lethality. These results are consistent with the discovery here of multiple different missense variants, which are likely to be less deleterious than truncating variants, in viable males with XLMR. CASK encodes a calcium/calmodulin-dependent serine protein kinase that is a member of the membrane-associated guanylate kinase (MAGUK) family and is located at the postsynaptic membrane of central nervous synapses23.
