We thank Dr. M. Downes for providing the α-skeletal actin promoter; Dr. G. D. Barish for comments on the manuscript; M.A. Lawrence for histology; Dr. S. Pfaff for the use of microscopes and photographic equipment; M. Lieberman and K.L. Schnoeker for photography; J.M. Shelton for advice on fiber staining; and E. Stevens and E. Ong for administrative assistance. YXW was supported by a postdoctoral fellowship from California Tobacco-Related Disease Research Program. JH was supported by the BK21 program, Ministry of Education, Korea. HK was supported by grant M1–0311-00–0145 from the Molecular and Cellular BioDiscovery Research Program, Ministry of Science and Technology, Korea. RME is an Investigator of the Howard Hughes Medical Institute at the Salk Institute for Biological Studies and March of Dimes Chair in Molecular and Developmental Biology. This work was supported by the Howard Hughes Medical Institute and the Hillblom Foundation.