FGF9 and craniosynostosis: Notably, a mouse semidominant mutant (Elbow knee synostosis; Eks) is caused by the amino acid substitution p.(Asn143Thr) in Fgf9 and manifests elbow joint fusions, knee joint dysplasia, and craniosynostosis [46,66,67]; importantly, variants in multiple members of the FGF receptor family cause several classical craniosynostosis syndromes, including Apert, Crouzon, Pfeiffer and Muenke syndromes [9].