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. Homotrimer. Forms a complex with activator 1 heteropentamer in the presence of ATP (By similarity). Component of the replisome complex (By similarity). Monoubiquitinated by the ube2b-rad18 complex on Lys-164. Monoubiquitination at Lys-164 also takes place in undamaged proliferating cells, and is mediated by the dcx(dtl) complex, leading to enhance PCNA-dependent translesion DNA synthesis (By similarity). Belongs to the PCNA family.