Acts as a transcriptional repressor (By similarity). Represses cAMP-responsive element (CRE)-mediated transcriptional activation (By similarity). 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 (By similarity). Plays a key role in the transcriptional regulation of T lymphocyte development (PubMed:22201126, PubMed:22753936). Homodimer (By similarity). Homodimer (PubMed:22753936). Interacts (via BTB domain) with TRIM28 (unphosphorylated or phosphorylated form) (By similarity). Localized in dot-like structures in the nucleus. Colocalized with SMRT in nuclear bodies. The sumoylated form is preferentially located in the nucleoplasm outside the nuclear bodies (By similarity). Expressed strongly in thymus and spleen, less in lymph nodes and peripheral blood mononuclear cells (PBMCs) and weakly in bone marrow. Strongly expressed in immature, but weakly in mature bone marrow-lymphocyte B. 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 (By similarity). 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-266. Sumoylation inhibits its transcriptional repression activity and regulates its subcellular localization (By similarity). Thymus of embryos show a small number of T-cell progenitors that are unable to progress through thymic differentiation. Adult mice show vestigial thymus and lymph nodes and a reduced spleen size. In the periphery and in the spleen, display an absence of mature T-cells, a reduced number of NK cells, but a normal number of mature B-cells.