Inhibits the activity of dimeric NF-kappa-B/REL complexes by trapping REL (RELA/p65 and NFKB1/p50) dimers in the cytoplasm by masking their nuclear localization signals (PubMed:1493333, PubMed:36651806, PubMed:7479976). On cellular stimulation by immune and pro-inflammatory responses, becomes phosphorylated promoting ubiquitination and degradation, enabling the dimeric RELA to translocate to the nucleus and activate transcription (PubMed:7479976, PubMed:7628694, PubMed:7796813, PubMed:7878466). {ECO:0000269|PubMed:1493333, ECO:0000269|PubMed:36651806, ECO:0000269|PubMed:7479976, ECO:0000269|PubMed:7628694, ECO:0000269|PubMed:7796813, ECO:0000269|PubMed:7878466}. This is the function of NFKBIA (NFKB inhibitor alpha, ENSG00000100906).