Craniosynostosis, Boston type is an autosomal dominant disorder that results in the premature fusion of cal-varial bones and ensuing abnormalities in skull shape. We showed previously that this disorder is tightly linked to the Msx2 homeobox gene on the long arm of chromosome 5, and that affected individuals bear a mutated copy of Msx2. In addition, transgenic mice in which either mutant or wild-type mouse Msx2 is overexpressed in the developing skull also exhibit craniosynostosis. That both mutant and wild-type Msx2 elicit craniosynos-tosis in transgenic mice and that the Boston type mutation is dominant led us to hypothesize that the mutation might enhance the normal function of Msx2. The mutation is located in position 7 of the N-terminal arm of the homeodomain, a region implicated in both target sequence recognition and protein-protein interactions. Here we test the hypothesis that the Pro148→ His mutation alters the DNA binding properties of Msx2. Using gel shift and binding site selection analyses, we show that the mutation enhances the affinity of Msx2 for a set of known Msx2 target sequences but has little or no effect on the site specificity ofMsx2 binding. The enhancement of Msx2 binding is due largely if not entirely to an increased stability of the mutant Msx2-DNA complex. These data provide a molecular-level explanation of how the Pro148→His mutation enhances Msx2 function and thus leads to the dominant craniosynostosis phenotype.