The membrane associated protein p120 Catenin (pp120 (Src substrate); p120cas) was identified as a tyrosine kinase substrate that is phosphorylated in Src transformed cells or in response to growth factor stimulation. It shares structural similarity with the Drosophila Armadillo protein and the vertebrate β-catenin and γ-catenin proteins. This similarity is evidenced by its characteristic Arm domain that is composed of 42-amino acid motif repeats. In the cell, p120 Catenin is localized to the E-Cadherin/catenins cell adhesion complex. Like β- and γ-catenin, p120 Catenin directly associates with the cytoplasmic C-terminus of E-Cadherin via its Arm domain and may similarly interact with other Cadherins. It exists as four isoforms that range in size from 90-115 kDa. Expression of these isoforms is heterogeneous in human carcinomas, suggesting that altered pp120 expression contributes to malignancy due to loss of functional cell adhesions. Multiple tyrosine residues (Y96, Y112, Y228, Y280, Y257, Y291, Y296, and Y302) in p120 Catenin are phosphorylated by Src and these phosphorylations may facilitate interaction with PTP1C/SHP-1 in response to EGF stimulation. Thus, p120 Catenin is an Arm domain protein that interacts with both cell adhesion molecules, such as cadherins and cell signaling molecules, such as PTP1C.