Negatively regulates class switch recombination or isotype switching in splenic B-cells. Homodimer (PubMed:33831416). Interacts (via the BTB domain) with HDAC1 and NCOR2 (PubMed:33831416). The BTB domain is important for homodimerization and for its function in negative regulation of class switch recombination. Belongs to the KIAA1841 family. Extended N-terminus.