Armadillo/beta-catenin signals in the nucleus--proof beyond a reasonable doubt?

Wnt signalling results in transcriptional stimulation of genes controlling normal and malignant development. A key effector of the canonical Wnt pathway is beta-catenin (also known as Drosophila melanogaster Armadillo (Arm)), thought to function as a nuclear co-activator of TCF transcription factors. This has been challenged by unexpected observations of membrane-bound Arm/beta-catenin signalling activity. Plausible explanations allow these observations to be reconciled with the large body of evidence supporting a nuclear function of Arm/beta-catenin.