(EN) Peptide variants corresponding to cancer specific isoform of proliferating cell nuclear antigen (caPCNA) interfere with intracellular protein-protein interaction, thereby causing a reduction in the proliferative potential of cancer. Cell-permeable caPCNA-derived peptides and their variants serve as therapeutic compositions to reduce the proliferation of cancer cells and also augment cytotoxic effects of existing cancer therapy. These compositions are also useful for chemoprevention of cancer.