We thank Gail Martin for the Cre construct (plasmid pML78) and Oliver Bogler for the IRES-hPLAP construct (plasmid 1726); Bjorn Olsen and Benoit de Crombrugghe for antibodies; the following individuals for in situ probe templates: Sophie Candille, Arend Sidow (Bmp2 and Bmp4), Vicki Rosen (Agg), and Bjorn Olsen (Col1a1); Véronique Lefebvre for Sox5 and Sox6 probe templates and useful discussions; Michelle Johnson for help with phenotypic assays on mice; Dr. Corrine Davis for help in evaluating synovial sections; Rebecca Rountree for Adobe Photoshop and Illustrator tips; and members of the Kingsley lab for helpful comments on the manuscript. This work was supported by an NIH predoctoral training grant (RR), a postdoctoral fellowship from the Arthritis Foundation (MS), and grants from the National Institutes of Health (DK). Dr. Kingsley is an associate investigator of the Howard Hughes Medical Institute.