In a normal developing child, skull bones are connected by fibrous sutures that allow pressure to be released as the brain grows and expands. Craniosynostosis is the premature fusion of these sutures in newborns and infants. In Saethre-Chotzen syndrome, a disorder characterized by synostosis of the coronal suture and other facial irregularities, heterozygous mutations in the basic helix-loop-helix (bHLH) transcription factors TWIST1 or TCF12 account for the majority of diagnoses. We hypothesize that Tcf12 functions as a suture-specific heterodimerization partner of Twist1 that promotes its ability to maintain osteoblast progenitors in an undifferentiated state. In support of such an interaction, loss of one allele of Tcf12 enhances coronal synostosis in Twist1+/- mice. In order to understand how early changes in osteoblast differentiation may account for suture defects in individual animals, we have developed a zebrafish model of Saethre-Chotzen syndrome by generating null alleles for tcf12 and the two zebrafish Twist1 homologs (twist1a and twist1b). Remarkably, one particular combination of alleles, tcf12-/-; twist1b-/-, survives to adulthood and displays a near complete and specific loss of the coronal suture. Whereas twist1a; twist1b double mutants have much earlier defects in specification of the neural crest-derived ectomesenchyme that generates the head skeleton, further loss of tcf12 partially restores early craniofacial development and allows survival to adulthood. These findings suggest that Twist1 may have different binding partners during ectomesenchyme and suture development, with Tcf12 acting as a Twist1 binding partner during later suture development. While mouse coronal sutures lie at a boundary between neural crest-derived frontal bones and mesoderm-derived parietal bones, zebrafish coronal sutures lie between the mesoderm-derived portions of the frontal and parietal bones. Our data thus provide strong evidence for deep evolutionary homology of the coronal suture between fish and mammals, as the role of Tcf12/Twist1 interactions is preserved regardless of tissue origin. We are presently conducting live imaging of osteoblast differentiation over time in our zebrafish model to better understand how early defects might result in later craniosynostosis.