The mammalian Rel/NF-kappaB family of transcription factors, including RelA, 
c-Rel, RelB, NF-kappaB1 (p50 and its precursor p105), and NF-kappaB2 (p52 and 
its precursor p100), plays a central role in the immune system by regulating 
several processes ranging from the development and survival of lymphocytes and 
lymphoid organs to the control of immune responses and malignant transformation. 
The five members of the NF-kappaB family are normally kept inactive in the 
cytoplasm by interaction with inhibitors called IkappaBs or the unprocessed 
forms of NF-kappaB1 and NF-kappaB2. A wide variety of signals emanating from 
antigen receptors, pattern-recognition receptors, receptors for the members of 
TNF and IL-1 cytokine families, and others induce differential activation of 
NF-kappaB heterodimers. Although work over the past two decades has shed 
significant light on the regulation of NF-kappaB transcription factors and their 
functions, much progress has been made in the past two years revealing new 
insights into the regulation and functions of NF-kappaB. This recent progress is 
covered in this review.