As a component of the trimeric and tetrameric DNA polymerase delta complexes (Pol-delta3 and Pol-delta4, respectively), plays a role in high fidelity genome replication, including in lagging strand synthesis, and repair. Pol-delta3 and Pol-delta4 are characterized by the absence or the presence of POLD4. They exhibit differences in catalytic activity. Most notably, Pol-delta3 shows higher proofreading activity than Pol-delta4. Although both Pol-delta3 and Pol-delta4 process Okazaki fragments in vitro, Pol-delta3 may also be better suited to fulfill this task, exhibiting near-absence of strand displacement activity compared to Pol-delta4 and stalling on encounter with the 5'-blocking oligonucleotides. Pol-delta3 idling process may avoid the formation of a gap, while maintaining a nick that can be readily ligated. Along with DNA polymerase kappa, DNA polymerase delta carries out approximately half of nucleotide excision repair (NER) synthesis following UV irradiation. Under conditions of DNA replication stress, required for the repair of broken replication forks through break-induced replication (BIR). Involved in the translesion synthesis (TLS) of templates carrying O6-methylguanine or abasic sites performed by Pol-delta4, independently of DNA polymerase zeta (REV3L) or eta (POLH). Facilitates abasic site bypass by DNA polymerase delta by promoting extension from the nucleotide inserted opposite the lesion. Also involved in TLS as a component of the POLZ complex. Along with POLD3, dramatically increases the efficiency and processivity of DNA synthesis of the minimal DNA polymerase zeta complex, consisting of only REV3L and REV7. Component of the tetrameric DNA polymerase delta complex (Pol-delta4), which consists of POLD1/p125, POLD2/p50, POLD3/p66/p68 and POLD4/p12, with POLD1 bearing DNA polymerase and 3' to 5' proofreading exonuclease activities. Following stress caused by DNA damaging agents or by replication stress, POLD4 is degraded and Pol-delta4 is converted into a trimeric form of the complex (Pol-delta3), which consists of POLD1, POLD2 and POLD3. Pol-delta3 is the major form occurring at S phase replication sites, as well as DNA damage sites. Also observed as a dimeric complex with POLD2 (Pol-delta2 complex). Component of the DNA polymerase zeta complex (POLZ), which consists of REV3L, MAD2L2, POLD2 and POLD3, with REV3L bearing DNA polymerase catalytic activity. Recruited to DNA damage sites within 2 hours following UV irradiation. Belongs to the DNA polymerase delta/II small subunit family.