Abstract: Fragments of Fen1 that interact with PCNA are disclosed, together with the use of such fragments or mimetics of Fen1 in methods of screening for compounds useful in treating disorders in which PCNA is implicated. In particular, substances which have the property of binding to PCNA are disclosed, said substances comprising: (i) a fragment of the Fen1 protein containing a peptide of 89 amino acids from the C-terminal region or an active portion thereof; or, (ii) a fragment of the Fen1 protein containing the sequence motif QGRLDxFF; or, (iii) a functional mimetic of said protein fragments; where “x” is preferably the selected form the amino acids S, D or G.

Abstract: Substances are disclosed which have the property of binding to PCNA, the substances comprising (i) a fragment of the p21WAF1 protein including residues 141 to 160 of the p21WAF1 amino acid sequence, or an active portion or derivative thereof; or (ii) functional mimetics of these protein fragments. In particular, the PCNA binding activity is shown to lie within the sequence motif OTSMTDFY, with the residues shown in bold being critical for PCNA binding, with those underlined being important. These substances are useful in the treatments of disorders in which PCNA is implicated, e.g. hyperproliferative disorders such as cancer and psoriasis, the substances binding to PCNA to inactivate it or functionally deplete its level. Also disclosed is the use of a yeast two hybrid screening technique for screening candidate peptides for binding to PCNA.