Gain-of-function mutations in fibroblast growth factor receptors have been identified in numerous syndromes associated with premature cranial suture fusion. Murine models in which the posterior frontal suture undergoes programmed fusion after birth while all other sutures remain patent provide an ideal model to study the biomolecular mechanisms that govern cranial suture fusion. Using adenoviral vectors and targeted in utero injections in rats, we demonstrate that physiological posterior frontal suture fusion is inhibited using a dominant-negative fibroblast growth factor receptor-1 construct, whereas the normally patent coronal suture fuses when infected with a construct that increases basic fibroblast growth factor biological activity. Our data may facilitate the development of novel, less invasive treatment options for children with craniosynostosis. Gain-of-function mutations in the fibroblast growth factor receptors (FGF-Rs) have been identified in many syndromes that have craniosynostosis as their defining characteristic including Apert, Pfeiffer, and Crouzon syndromes. These syndromes are all characterized by craniosynostosis (ie, premature fusion of one or more cranial sutures) and varying degrees of extracranial skeletal abnormalities. 1 In vitro studies of these mutated FGF-Rs have identified at least two biomolecular mechanisms that mediate excess signaling by these receptors: 1) formation of receptor dimers in the absence of receptor ligand [ie, basic fibroblast growth factor (FGF), FGF-2] resulting in ligand-independent intracellular signaling; and 2) increased affinity of mutated receptors for ligand. 2-6 Increased FGF-biological activity is thought to lead to the premature fusion of cranial sutures and the dysmorphic craniofacial phenotype associated with these syndromes. To date, however, direct evidence supporting this hypothesis has been lacking.To understand the events that lead to premature cranial suture fusion, numerous investigators have relied on animal models of cranial suture fusion. Murine suture development, in which the posterior frontal (PF) suture undergoes programmed sutural fusion shortly after birth, provides an ideal model to study the biomolecular mechanisms that occur before, during, and after cranial suture fusion. 7,8 In these models, the PF cranial suture fuses between postnatal days 12 to 22 in the rat and days 25 to 45 in the mouse whereas all other sutures, including the coronal (COR) and sagittal (SAG), remain patent ( Figure 1). Using these models, our group and others have demonstrated that the dura mater directly underlying a cranial suture regulates sutural fate (ie, fusion or patency). 8 -11 In addition, we have shown that FGF-2 mRNA and protein expression in the fusing PF suture is up-regulated in the suture-associated dura mater just before and during active sutural fusion. 12,13 These findings implicate FGF biological activity in the regulation of cranial suture fusion.In this study, replication-deficient adenoviruses encoding a truncated form of FGF-R1 (AdCA...