PCNA (proliferating cell nuclear antigen) is a sliding clamp that plays important roles during DNA replication and repair. In yeast, PCNA can be modified by either mono- or poly-ubiq uitin or by addition of SUMO moieties. These different modifications direct the activity of PCNA toward alternative DNA transactions. In mammals, PCNA ubiquitination was reported, and it seems to have similar effects to those observed in yeast. However, for a long time, no SUMOylation of PCNA could be detected. Two recent papers report the detection of SUMOylated PCNA in mammalian cells. Here, we summarize similarities and differences between the various biological systems and present the current view of the way by which PCNA modification can affect DNA replication and repair pathways.