Pygopus-1 and -2 are nuclear transcription co-activators. Structurally they are characterized by an N-terminal homology domain and C-terminal PHD (plant homeodomain) finger Zn2+-binding domain. The PHD finger binds to legless/Bcl9, which in turn binds beta-catenin, a downstream mediator of Wnt signaling. This complex binds TCF/LEF DNA binding proteins. Formation of the quaternary complex is required for the activation of Wnt-responsive genes.