RNA in situ hybridization was performed as described (Storm and Kingsley 1996), with the following modifications: (1) Prior to the acetylation step, sections were incubated with 10–20 μg/ml proteinase K for 30 s to 7 min at room temperature (depending on the developmental stage), followed by refixing in 4% PFA and washing three times in PBS; (2) prehybridization step was skipped, and (3) embryonic tissue sections used a different color development mix (Thut et al. 2001). Probes for the following genes have been published previously: Bmpr1a (Mishina et al. 1995), Col2a1 (Metsaranta et al. 1991), Col10a1 (Apte et al. 1992), Gdf5 (Storm and Kingsley 1996), Osteocalcin (Celeste et al. 1986), and Sox5 and Sox6 (Lefebvre et al. 1998). The following probe templates were gifts: Agg, Dr. Vicki Rosen, Genetics Institute; Bmp2 and Bmp4, Arend Sidow, Stanford University; Col1a1, Bjorn Olsen, Harvard Medical School; Bmpr1b, Col3a1, and Mat4 probes were made from ESTs with IMAGE clone numbers 5056341, 478480, and 406027, respectively (Invitrogen, Carlsbad, California, United States).