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).