The invention provides structurally-constrained peptides by hydrocarbon stapling of a BCL9 HD2 helix for use as a therapeutic agent. The invention further provides methods and kits for use of the structurally-constrained peptide of the instant invention. The invention is based, at least in part, on the results provided herein demonstrating that hydrocarbon stapled helical peptides display excellent proteolytic, acid, and thermal stability, restore the native helical structure of the peptide, possess superior pharmacokinetic properties compared to the corresponding unmodified peptides, and are highly effective in binding to β-catenin in vitro, in cellulo, and in vivo, disrupting the BCL-9/β-catenin interaction, and thereby interfering with deregulated Wnt/β-catenin signaling for therapeutic benefit in a variety of human diseases including human cancer.