Monoubiquitination of proliferating cell nuclear antigen (PCNA) is a critical posttranslational modification essential for DNA repair by translesion DNA synthesis (TLS). The Rad18 E3 ubiquitin ligase cooperates with the E2 Rad6 to monoubiquitinate PCNA in response to DNA damage. How PCNA is monoubiquitinated in unperturbed cells and whether this plays a role in the repair of DNA associated with replication is not known. We show that the CRL4Cdt2 E3 ubiquitin ligase complex promotes PCNA monoubiqutination in proliferating cells in the absence of external DNA damage independent of Rad18. PCNA monoubiquitination via CRL4Cdt2 is constitutively antagonized by the action of the ubiquitin-specific protease 1 (USP1). In vitro, CRL4Cdt2 monoubiquitinates PCNA at Lys164, the same residue that is monoubiquitinated by Rad18. Significantly, CRL4Cdt2 is required for TLS in nondamaged cells via a mechanism that is dependent on PCNA monoubiquitination. We propose that CRL4Cdt2 regulates PCNA-dependent TLS associated with stresses accompanying DNA replication. © 2010 Elsevier Inc. All rights reserved.