The vertebrate skull functions as a protective barrier while adjusting to the expansion of the brain during development. The transcription factor FoxC1 plays an essential role in tissue interactions that pattern the cranium as illustrated by the fact that FoxC1 null mice lack calvarial bones. Motif scans for possible kinases that may phosphorylate FoxC1 identified p38 MAPK and ERK1 as potential candidates. Objective: The aim of this study was to determine if FoxC1 is regulated by phosphorylation and to identify potential kinases involved in this regulation. Methods: To determine if FoxC1 is phosphorylated, we used lambda protein phosphatase (λ-PPASE), a protein phosphatase with activity towards phosphorylated serine, threonine, and tyrosine residues. Xenopus embryos injected at the 2-cell stage with 500 pg of HA- FoxC1 mRNA, were lysed at stage 10.5. After an HA immunoprecipitation, the matrix was incubated in the presence or absence of λ-PPASE. Eluted proteins were detected by immunoblotting with an anti-HA antibody. In 293T cells transfected with HA-FoxC1, p38 MAPK inhibition was achieved by using SB203580 (10μM, 16h); 293T cells were also co-transfected with MAPK phosphatase 1 (MKP-1). Results: λ-PPASE incubation caused a shift in the molecular weight of HA-FoxC1 in Xenopus embryos, indicating that FoxC1 is phosphorylated in vivo. Furthermore, in 293T cells, inhibition of p38 MAPK with SB203580 or co-transfection with MKP-1 resulted in faster-migrating forms of FoxC1, providing evidence that p38 or a related MAPK family member is responsible for FoxC1 phosphorylation. Conclusion: FoxC1 is phosphorylated in vivo possibly by a member of the MAPK family. This observation will be extended in future studies to include experiments aimed at the precise identification of the kinase(s), the FoxC1 sites essential for this phosphorylation and ultimately, the functional significance of this post-translational modification. Funding: DE-14528 (TD), NIH-HD29468