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Wnt proteins are secreted morphogens that are required for various developmental processes including cell fate determination, cell proliferation and cell death. After binding to the Frizzled receptor, three pathways are triggered: the canonical pathway, the planar cell polarity pathway and the Wnt/Ca2+ pathway. In the canonical pathway, the major effector β-catenin enters the nucleus and promotes Wnt-regulated gene expression. The planar cell polarity pathway promotes cytoskeleton remodeling and cell movement via activation of RhoA and Rac. The Wnt-Ca2+ pathway mediates multiple functions via an increase in transient Ca2+ levels and the subsequent activation of PKC, CAMKII and calcineurin. Wnt pathways not only facilitate various aspects of embryogenesis, including apical/basal polarity and neural tube patterning, but also induce tumorigenesis. Wnt signaling has also recently been reported to regulate stem cell differentiation.