NF-κB subunit structures and dimeric combinations. All subunits contain a Rel homology domain (RHD). NFKB1 and NFKB2 have a glycine rich region (GRR), followed by an Ankyrin repeat domain (ANK) in the precursors p105 and p100. p65, RelB, and c-Rel contain a transactivation domain (TAD), with RelB additionally containing a leucine zipper (LZ) motif. While most dimers activate transcription, p50:p50, p50:p52, and p52:p52 dimers lack a TAD, and therefore repress transcription in the absence of co-activating factors and p65:RelB, c-Rel:RelB, and RelB:RelB dimers are thought to have limited DNA binding [8,9,10].