Pygo1 homozygous null mice were viable and fertile with no developmental defects detectable. This was surprising given the importance of the pygopus gene in Wnt signaling in Drosophila, and the reported expression during development of the mouse Pygo1 gene in, for example, the brain, limbs, kidney and branchial arches [14,16] (Yu J, Valerius MT, McMahon AP, contribution to GUDMAP, ).