Model of the action of SOXC proteins in early skeletogenesis. SOXC proteins cell-autonomously block the activity of GSK3 in perichondrium and presumptive joint mesenchymal cells and thereby synergize with canonical WNT signaling. Enhanced stabilization of β-catenin along with a possible independent action of the SOXC proteins leads to Sox9 repression. SOXC proteins thereby critically contribute to the proper delineation and articulation of the cartilage primordia of the developing vertebrate skeleton.