Double-homozygous mutant Pygo1 and Pygo2 mice had a phenotype similar to that of single Pygo2 nulls (Figure 1, Table 1). There was no significant synergism of developmental abnormalities in the double mutants. Together these results suggest that Pygo2 is required for the proper development of a limited number of structures, whereas Pygo1 is not necessary for normal development.