Acts as a transcriptional repressor (PubMed:20797634). Represses cAMP-responsive element (CRE)-mediated transcriptional activation (PubMed:21706167). In addition, has a role in translesion DNA synthesis. Requires for UV-inducible RAD18 loading, PCNA monoubiquitination, POLH recruitment to replication factories and efficient translesion DNA synthesis (PubMed:24657165). Plays a key role in the transcriptional regulation of T lymphocyte development (By similarity). Homodimer. Homooligomer. Isoform 1 and isoform 2 can homodimerize. Heterodimer of isoform 1 and isoform 2. Interacts (via BTB domain) with TRIM28 (unphosphorylated or phosphorylated form) (PubMed:24657165). Localized in dot-like structures in the nucleus (PubMed:21706167). Colocalized with SMRT in nuclear bodies (PubMed:20797634). The sumoylated form is preferentially located in the nucleoplasm outside the nuclear bodies(PubMed:20797634). Both the BTB domain and C2H2-type motifs are necessary for transcriptional repression activity. The BTB domain is also necessary for oligomerization and efficient sumoylation. The hydrophobic cluster preceding Lys-328 enhanced sumoylation efficiency (PubMed:20797634). The UBZ-type zinc finger domain is required for targeting ZBTB1 to UV damage sites and for PCNA monoubiquitination. UBZ-type zinc finger domain mediates binding to 'Lys-63'-linked polyubiquitin chains (in vitro). Sumoylated with SUMO2 at Lys-328 and to a lesser extent at Lys-265. Sumoylation inhibits its transcriptional repression activity and regulates its subcellular localization. Extended N-terminus.