Mutations in human CASK can lead to a number of neurological diseases including FG syndrome 4, X-linked mental retardation with or without nystagmus and intellectual disability and microcephaly with pontine and cerebella hypoplasia (Froyen et al., 2007; Najm et al., 2008; Piluso et al., 2009; Tarpey et al., 2009). Here, CASK is linked to Nystagmus.