Beta-catenin is a widely expressed 90-kDa protein with dual functions in cell adhesion and Wnt signalling. At the membrane, beta-catenin forms complexes with E-cadherin to generate cell adhesion complexes responsible for maintaining the structural integrity of many epithelial tissues. On the other hand, accumulation of beta-catenin in the nucleus in response to Wnt signalling facilitates complex formation with Tcf transcription factors, leading to activation of a genetic program influencing a range of cellular processes including cell growth, cell movement, and cell fate. Chronic activation of the Wnt signalling pathway as a result of mutations in key pathway components, including beta-catenin itself, is a major cause of cancer. The associated increase in nuclear beta-catenin protein is therefore considered to be a hallmark of Wnt-driven cancers and an invaluable tool to detect active Wnt signalling.