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The Wnt signal transduction pathway. (A) The Wnt protein binds to its receptor, a member of the Frizzled family of proteins. The Frizzled protein then activates Disheveled, allowing it to become an inhibitor of glycogen synthase kinase 3 (GSK-3). GSK-3, if it were active, would prevent the dissociation of β-catenin from the APC protein. So, by inhibiting GSK-3, the Wnt signal frees β-catenin to associate with an LEF protein to become an active transcription factor. (B) In adult cells, if the gene for APC or β-catenin is mutated such that they cannot bind together, β-catenin is constitutively allowed into the nucleus. This causes it to activate certain cell division genes and initiate tumors. (B after Pennisi 1998.)