The NF-kB family of induced-activated transcription factors are dimers comprised of members of the NF-kB /rel protein family. Activation of NF-kB is induced by a remarkably large number of stimuli including viruses, TNF, IL-1, PMA , LPS, UV light, and others. In most cell types, NF-kB is present in the cytoplasm as a 50kD (p50) subunit and a 65kD (p65, rel A) subunit. In unstimulated cells, nuclear localization signals present on p65 are masked by members of the Ik-B family of inhibitory proteins. Subsequent to cell stimulation, IkB undergoes phosphorylation, ubiquitination and degradation by a proteosome-dependent pathway, allowing nuclear translocation of the active dimeric NF-kB transcription factor. In the nucleus, NF-kB binds to consensus sequences where, along with other co-factors, it participates in activating or enhancing the expression of specific genes.