Alk5 (and Alk2) mutant and control embryos were generated by mating Alk5ko/+(Alk2ko/+)/Wnt1-Cre male mice with females homozygous for the Alk5flox (Alk2flox) allele and the R26R reporter[12,19]. Genotyping was performed by PCR as described earlier [21,42]. Wnt1-Cre mice were kindly provided by A. McMahon (Harvard University) and R26R reporter mice were obtained from the Jackson laboratories. All studies were carried out at the Animal Care Facility of the Saban Research Institute of Childrens Hospital Los Angeles in accordance with institutional guidelines.