This protein is an auxiliary protein of DNA polymerase delta and is involved in the control of eukaryotic DNA replication by increasing the polymerase's processibility during elongation of the leading strand. Involved in DNA repair. Homotrimer (PubMed:15201901). Interacts with RAD30 (PubMed:11545742). Interacts with MCM10 (PubMed:16782870). Interacts with UBP10 (PubMed:22829782). Sumoylated on Lys-164, and to a lesser extent on Lys-127 by the UBC9/SIZ1 complex during S-phase; which impairs ubiquitination and function in DNA repair. Monoubiquitinated on Lys-164 by the UBC2/RAD18 complex upon DNA damage, and then polyubiquitinated through 'Lys-63'-linkage by UBC13/MMS2. Ubiquitination is required for UBC2-mediated DNA repair. Lys-164 is deubiquitinated by UBP10 (PubMed:22829782). Belongs to the PCNA family.