A transcription factor involved in rapid cellular responses to stresses, such as injury or invading pathogens, and in many aspects of cell proliferation and survival, including programmed cell death (apoptosis). It is found in the cells of organisms ranging from flies to mammals and has a key role in such disorders as systemic shock, inflammation, and cancer. The name derives from the finding that B cells require this factor in order to transcribe the κ‎ light chain gene (hence nuclear factor for κ‎ light chain transcription in B cells). Many different extracellular signals can lead to activation of NFκ‎B, notably signal pathways involving Toll-like receptors, which mediate early immune responses to pathogens. NFκ‎B is a heterodimer of two protein subunits, both of which have DNA-binding domains. In its inactive state in the cytoplasm it is bound by an inhibitor called Iκ‎B. In response to a signal the latter is phosphorylated, marking it for degradation and enabling the freed NFκ‎B to migrate to the cell nucleus, where it ‘switches on’ the appropriate genes to produce the cellular response. Faulty control of NFκ‎B signalling is a crucial factor in certain human tumours, because genes promoting cell proliferation remain switched on.