NF-KB is a ubiquitous transcription factor. Nevertheless, its properties seem to be most extensively exploited in cells of the immune system. Among these properties are NF-KB's rapid posttranslational activation in response to many pathogenic signals, its direct participation in cyto plasmic/nuclear signaling, and its potency to activate transcription of a great variety of genes encoding immunologically relevant proteins. In vertebrates, five distinct DNA binding subunits are currently known which might extensively heterodimerize, thereby forming complexes with distinct transcriptional activity, DNA sequence specificity, and cell type-and cell stage-specific distribution. The activity of DNA binding NF-KB dimers is tightly controlled by accessory proteins called IKB subunits of which there are also five different species currently known in vertebrates. IKB proteins inhibit DNA binding and prevent nuclear uptake of NF-KB complexes. An exception is the Bcl-3 protein which in addition can function as a transcription activating subunit in the nucleus. Other IKB proteins are rather involved in terminating NF-KB's activity in the nucleus. The intra cellular· events that lead to the inactivation of IKB, ie the activation of NF-KB, are complex. They involve phosphorylation and proteolytic reactions and seem to be controlled by the cells' redox status. Interference with the activation or activity of NF-KB may be beneficial in suppressing