We thank L. Tessarollo for providing NT-3 KO animals, L. Reichardt for TrkA and TrkC antibodies, F. Stockdale for the S46 antibody, K. Muneoka for sepharose beads, and B. King for maintenance of animals used in this study. We particularly thank E. Frank for many helpful comments and discussions about this study and for sharing unpublished data from his laboratory. Supported by National Institutes of Health/National Institute of Dental and Craniofacial Research grant P01 DE07734.