Proteins encoded by the v-Rel viral oncogene and its cellular homolog, c-Rel, are members of a family of transcription factors that include the two subunits of the transcription factor NFkappaB (p50 and p65) and the Drosophila maternal morphogen, dorsal. Both proteins specifically bind to DNA sequences that are the same or slight variations of the 10 bp kappaB sequence in the immunoglobulin kappa light chain enhancer. This same sequence is also present in a number of other cellular and viral enhancers. The DNA binding activity of NFkappaB is activated and NFkappaB is subsequently transported from the cytoplasm to the nucleus in cells exposed to mitogens or growth factors. cDNAs encoding precursors for two distinct proteins have been described, designated p105 and p100. The p105 precursor contains p50 at its amino-terminus and a C-terminal region that when expressed as a separate molecule, designated PDI, binds to p50 and regulates its activity.The NFkappaB transcription factor is a protein complex consisting of a DNA binding subunit and an associated protein. The DNA binding subunit, also referred to as RelA, is functionally related to c-Rel p75 and RelB p68. NFkappaB p65 is phosphorylated at Serine 276 as a response to TNF.