Contributes to the regulation of transcriptional activation of NF-kappa-B target genes. In the cytoplasm, inhibits the nuclear translocation of the NF-kappa-B p50 subunit (By similarity). In the nucleus, acts as transcriptional activator that promotes transcription of NF-kappa-B target genes. Contributes to the regulation of cell proliferation. Component of a complex consisting of the NF-kappa-B p52-p52 homodimer and BCL3. Component of a complex consisting of the NF-kappa-B p50-p50 homodimer and BCL3. Interacts with N4BP2, COPS5 and PIR (By similarity). Interacts with CYLD. Ubiquitination via 'Lys-63'-linked ubiquitin chains is required for nuclear accumulation. Polyubiquitinated. Ubiquitination via 'Lys-63'-linked ubiquitin chains is required for nuclear accumulation. Deubiquitinated by CYLD, which acts on 'Lys-63'-linked ubiquitin chains. Deubiquitination by CYLD prevents nuclear accumulation. Activated by phosphorylation. It is uncertain whether Met-1 or Met-9 is the initiator. Truncated N-terminus.