Novel compounds that have been found effective in inhibiting PDZ domain interactions, and particularly interactions of PDZ domains in MAGIs with the oncogenic (tumor suppressor) protein PTEN and interactions between the PDZ domain in the Dishevelled (Dvl) protein and other proteins such as the Frizzled (Fz) protein, have the general formula (I) or (III). The invention also includes combinatorial libraries, arrays and methods for screening and studying proteins using such compounds. Compounds of the invention have produced apoptosis in certain cell lines that overexpress the Dishevelled protein (Dvl), inhibiting Wnt signaling.