Acknowledgements
We thank A. McMahon for providing the Wnt1-Cre mouse line, N. Manley for the PTH probe and B. Choudhary for Tgfbr2/Wnt1-Cre embryos. H.M.S. was supported by grants from the NIH, and V.K. by grants from the Robert E. Schneider Foundation and the NIH (HL074862 and DE013085).