The p14ARF tumour suppressor regulates a series of cell cycle regulatory proteins to promote cellcycle arrest in response to abnormal hyperproliferative growth stimuli. p14ARF alterations arecommon in human cancers and, when inherited, confer susceptibility to cutaneous melanoma. Wenow propose that the mechanism of p14ARF action may involve the covalent modification of itsbinding partners with the small ubiquitin-related protein SUMO-1. In particular, we demonstratethat p14ARF interacts with the SUMO E2 conjugating enzyme, Ubc9 and enhances the sumoylationof its binding partners, hdm2, E2F-1, HIF-1?, TBP-1 and p120E4F. Furthermore, p14ARF-inducedsumoylation is abrogated by a subset of melanoma-associated p14ARF mutations. These resultsprovide a mechanism for p14ARF action through a common modification of diverse bindingpartners.