Accessory component of both the DNA polymerase delta complex and the DNA polymerase zeta complex. 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 DNA polymerase zeta complex. Along with POLD3, dramatically increases the efficiency and processivity of DNA synthesis of the DNA polymerase zeta complex compared to the minimal zeta complex, consisting of only REV3L and REV7. Component of both the DNA polymerase delta and DNA polymerase zeta complexes. 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. Within Pol-delta4, directly interacts with POLD1, POLD3 and POLD4. 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). Pol-delta2 is relatively insensitive to the PCNA stimulation (2-5-fold) compared to Pol-delta4 that is stimulated by over 50-fold. Contrary to the other components of Pol-delta4, does not directly interact with PCNA. As POLD1 and POLD4, directly interacts with WRNIP1; this interaction stimulates DNA polymerase delta-mediated DNA synthesis, independently of the presence of PCNA. This stimulation may be due predominantly to an increase of initiation frequency and also to increased processivity. Directly interacts with POLDIP2 and POLDIP3. Directly interacts with KCTD13/PDIP1; in the presence of PCNA, this interaction may stimulate DNA polymerase activity. Component of the tetrameric Pol-zeta complex (Pol-zeta4), which consists of REV3L, MAD2L2, POLD2 and POLD3, with REV3L bearing DNA polymerase catalytic activity (By similarity). Interacts with KCTD10 (PubMed:15982757). Recruited to DNA damage sites within 2 hours following UV irradiation. Belongs to the DNA polymerase delta/II small subunit family.