Beta-catenin is essential for cadherin-based cell adhesion and Wnt/Wingless growth factor signaling. In these roles, it binds to cadherins, Tcf-family transcription factors, and the tumor suppressor gene product Adenomatous Polyposis Coli (APC). A core region of beta-catenin, composed of 12 copies of a 42 amino acid sequence motif known as an armadillo repeat, mediates these interactions. The three-dimensional structure of a protease-resistant fragment of beta-catenin containing the armadillo repeat region has been determined. The 12 repeats form a superhelix of helices that features a long, positively charged groove. Although unrelated in sequence, the beta-catenin binding regions of cadherins, Tcfs, and APC are acidic and are proposed to interact with this groove.