Cleft palate is a common birth defect in humans and is a common phenotype associated with syndromic mutations in fibroblast growth factor receptor 2 (Fgfr2). Cleft palate occurred in nearly all mice homozygous for the Crouzon syndrome mutation C342Y in the mesenchymal splice form of Fgfr2. Mutant embryos showed delayed palate elevation, stage-specific biphasic changes in palate mesenchymal proliferation, and reduced levels of mesenchymal glycosaminoglycans (GAGs). Reduced levels of feedback regulators of FGF signaling suggest that this gain-of-function mutation in FGFR2 ultimately resembles loss of FGF function in palate mesenchyme. Knowledge of how mesenchymal FGF signaling regulates palatal shelf development may ultimately lead to pharmacological approaches to reduce cleft palate incidence in genetically predisposed humans.Crouzon syndrome | fibroblast growth factor receptor 2 | cell proliferation | cell surface receptor | glycosaminoglycan C raniosynostosis syndromes, as well as syndromic and nonsyndromic cleft palate, have been associated with fibroblast growth factor receptor (FGFR) mutations. Of the four highly conserved FGFRs, FGFR2 is the most commonly mutated FGF receptor (1-4). The FGF-FGFR family is involved in multiple intracellular signaling mechanisms in embryonic development, cell growth, wound healing, and tumorigenesis. The FGFRs are receptor tyrosine kinases that signal via a ternary complex of FGFR, FGF, and glycosaminoglycans (GAGs) (5). The ternary complex formation leads to receptor dimerization, autophosphorylation, and activation of downstream signaling cascades (6).FGF-FGFR signaling is active during palatogenesis, and genetic FGF mutations may contribute to 5% of cases of nonsyndromic cleft lip and palate (7). In children with isolated cleft palate, mutations in Fgfr1, Fgfr2, Fgfr3, and Fgf8 have been identified. Additionally, single nucleotide polymorphisms in children with nonsyndromic cleft lip and palate are associated with Fgf3, Fgf7, Fgf10, Fgf18, and Fgfr1, providing confirmation of the importance of the FGF-FGFR system in palate development (7). Knockout mouse models for Fgf10 and Fgfr2b develop cleft palate (8, 9), establishing the necessity of epithelial FGF signaling in normal palatogenesis. Cleft palate in Fgf18 −/− mice (10) suggests that mesenchymal FGF signaling may also be important for palate development.Cleft palate has been associated with both gain-of-function and loss-of-function FGFR mutations (7,(11)(12)(13)(14)(15). Why gain-of-function or loss-of-function mutations result in the same palatal phenotype remains unclear, and understanding this phenomenon may provide additional insight into the pathogenesis of cleft palate. To address this question, we have focused on Crouzon syndrome (CS), a disorder resulting from a missense Fgfr2 mutation that displays an increased incidence of cleft palate in humans (14).CS occurs with an incidence of 12.5 per million births and results from genetic gain-of-function mutations in Fgfr2 (1). Craniofacial anomalies are its hallmark...