BackgroundTranscription factors of the nuclear factor κ B (NF-κB)/Rel family is a ubiquitously expressed transcription factor that regulates many cytokine and Ig genes. It is involved in immune, inflammatory, viral, and acute phase responses. There are five family members in mammals. RelA (p65), c-Rel, RelB, NF-κB1 (p105/p50) and NF-κB2 (p100/p52). The most studied NF-κB complex consists of the p50 and p65 subunits, both containing a 300 amino acid region with homology to the Rel proto-oncogene product. The p50 subunit binds DNA, whereas the p65 subunit is responsible for the interaction of NF-κB with its inhibitor, IκB. In most cell types, the p50/p65 heterodimer is located within the cytoplasm complexed to IκB. This complex prevents nuclear translocation and activity of NF-κB. In response to stimuli such as cytokines, LPS, and viral infections, IκB is phosphorylated at critical residues. This phosphorylation induces dissociation of the IκB/NF-κB complex, allowing the free heterodimeric NF-κB to form a heterotetramer that translocates to the nucleus. In the nucleus, it binds to the κB site within promoters and enhancers and functions as a transcriptional activator.