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β-Catenin is a multifunctional protein that is both an integral component of adherens junctions and a pivotal member of a signal transduction pathway. The cytoplasmic pool of β-catenin, which participates in signal transduction, is highly regulated. Binding to the adenomatous polyposis coli tumor suppressor protein can stimulate the degradation of β-catenin, whereas signaling initiated by the extracellular Wnt-1 oncoprotein or selected mutations in β-catenin itself results in the accumulation of higher levels of β-catenin in the cytoplasm. A variety of experiments from several model systems have converged to elucidate the mechanisms involved in this regulation as well as the downstream effectors of β-catenin. These studies have recently been extended to demonstrate that deregulation of this pathway contributes to cancer in humans.