A dramatic example of the role of Eph and ephrins in cell sorting, as well as the importance of proper cell sorting during development, was recently provided by the analysis of the phenotypes exhibited by ephrin-B1 heterozygous female mice [7,8] and by the identification of mutations in the ephrin-B1 gene in human craniofrontonasal syndrome (CFNS) patients [9,10]. This evidence concerns the gene EPHA1 and craniofrontonasal syndrome.