Abnormal canonical Wnt signaling causes developmental abnormalities and increased Wnt signaling drives the formation of a variety of tumors, including the childhood medulloblastoma and the vast majority of human colorectal carcinomas. This has generated considerable interest in the identification of small molecule inhibitors of the Wnt pathway for therapy. Recently, it has been suggested that small molecules that inhibit the Wnt pathway through stabilization of Axin, such as IWR-1 and XAV939, would be useful in therapy. Axin is a key negative regulator of the Wnt pathway because of its role as a scaffold of the ?-catenin destruction complex, which keeps the pathway off in the absence of ligand. We recently discovered that stabilization of Axin through either mutation or by treatment with IWR-1 has the opposite effect of promoting Wnt signaling in certain stem/progenitor populations during development. These findings raise the possibility that the circuitry of the canonical Wnt pathway is different in stem cells. We will test the hypothesis that stabilized Axin can activate the Wnt pathway by favoring the formation of an Axin-LRP5/6 membrane complex that increases the level of free ?-catenin. We will define the mechanisms responsible for the cell type specificity of the effects of stabilized Axin protein. We will use a genetic approach to identify other cells in the mouse where stabilization of Axin2 protein activates the canonical Wnt pathway, focusing on the Wnt-dependent stem cells in the intestine, brain, breast and skin that could act as cancer stem cells. The vast majority of cases of human colorectal carcinoma are associated with mutation or silencing of APC, a negative regulator of the Wnt pathway. We will test whether the stabilized allele of Axin2 suppresses or enhances intestinal polyp formation in the mouse ApcMin model. The studies will provide a paradigm for how stem/progenitor cells differ from other cells in their sensitivity to Wnt pathway inhibitors, which will define appropriae targets for therapy.