Proliferating Cell Nuclear Antigen (PCNA) is a sliding clamp protein that controls processive DNA replication. Higher levels of DNA replication required by cancer cells, results in PCNA being upregulated in almost 90% of cancers. Inhibition of PCNA at both mRNA and protein levels, has been shown previously to reduce cell proliferation, and inhibitors have shown selective toxicity toward malignant cells in both cell culture and mouse models, although the mechanism is not clear. PCNA has therefore been put forth as a target for next generation cancer therapeutics. Here we investigate constrained peptides derived from the naturally occurring replication inhibitor protein p21, as a rapid route to novel inhibitors of PCNA. P21 binds to PCNA in a 3.10 helical conformation, so model peptides have been modified to incorporate side-chain/side-chain covalent constraints in an attempt to stabilise the bound conformation, improving potency, stability and cellular uptake. Using NMR spectroscopy, we show that our constrained peptides have better defined helical structures in solution than the unconstrained p21 peptide, and that these structures align well with the crystal structure of p21 in complex with PCNA. We have solved the crystal structure of one of the constrained peptides in complex with PCNA, revealing the bound conformation to be similar to that observed for the parent peptide. Thus, covalent constraint of p21 peptide side-chains results in stabilisation of 3.10 helical structure, and we are now looking to determine the effect of these constraints on PCNA binding affinity using a recently developed fluorescence polarization assay. Finally, experiments are underway to examine the cellular uptake of these peptides in different cell types, and their effect on breast cancer cell replication.