beta-Catenin is a key downstream effector in the Wnt signaling pathway. It is implicated in two major biological processes in vertebrates: early embryonic development and tumorigenesis. CK1 phosphorylates beta-catenin at Ser45. This phosphorylation event primes beta-catenin for subsequent phosphorylation by GSK-3beta. GSK-3beta destabilizes beta-catenin by phosphorylating it at Ser33, Ser37, and Thr41. Mutations at these sites result in the stabilization of beta-catenin protein levels and have been found in many tumor cell lines.