A transgenic mouse insertional mutant displayed the phenotype of altered cranial morphology with sex-linked cleft palate. We have cloned the disrupted genomic X-linked locus and report the identification of the mCASK gene. The gene is transcribed to produce two messages of 4.5 and 9.5 kb expressed during development and in adult tissues, particularly the brain. We describe the isolation of two differentially spliced mouse cDNAs from the locus (mCASK-A and mCASK-B). The mCASK-B cDNA probably represents the full-length product of the 4.5-kb transcript. The identical N-termini of the predicted encoded proteins (mCASK-A and -B) are highly homologous to Ca2+/calmodulin-dependent protein kinase II, while the deduced C-terminus of mCASK-B is highly homologous to a family of multidomain proteins containing a guanylate kinase motif, the MAGUK proteins. mCASK-B is a new member of an emerging family of genes in which the encoded proteins combine these domains, termed here, the CAMGUKs, including rat CASK, Caenorhabditis elegans lin-2, and Drosophila caki/camguk. The CAMGUKs are likely to be effectors in signal transduction as regulatory partners of transmembrane molecules, modulated by calcium and nucleotides. The transgene in this mutant mouse line integrated into an intron that bisects the encoded calmodulin-binding domain, a potentially important regulatory domain of the predicted protein, generating hybrid transcripts.