Protein inhibitors of activated STATs (PIAS) represent a small family of nuclear proteins that modulate the activity of many transcription factors and act as E3 ligases for covalent modification of proteins with the small ubiquitin-like modifier (SUMO). In particular, PIASy has been shown to inhibit the activation of gene expression by the IFN-responsive transcription factor STAT1 and the Wnt-responsive transcription factor LEF1. To assess the function of PIASy in vivo, we generated and analyzed mice carrying a targeted mutation of the Piasy gene. We find that homozygous mutant mice have no obvious morphological defects and have a normal distribution of lymphocyte populations. Molecular analysis of signaling in response to IFN-gamma and Wnt agonists revealed a modest reduction in the activation of endogenous and transfected target genes. Two-dimensional analysis of total proteins and SUMO-modified proteins in transformed pre-B cells showed no significant differences between wild-type mice and homozygous mutant mice. Taken together, our data indicate that PIASy has a modest effect on cytokine and Wnt signaling, suggesting a redundancy with other members of the family of PIAS proteins.