NFkB1 (NFkB p50) and NFkB2 (NFkB p52) are class I members of the Rel/NFkB family of transcription factors that also includes RelA, c-Rel and RelB. Rel/NFkB members regulate expression of genes that participate in immune, apoptotic and oncogenic processes. Class I members lack a C-terminal transcriptional activation segment found in class II members, but contain an RHD (Rel homology domain) that mediates DNA binding, nuclear localization and dimerization. NFkB1 and NFkB2 105 and 100 kDa precursors are proteolytically cleaved to 50 and 52 kDa active subunits, respectively. NFkB is predominantly localized in the cytoplasm as a complex with inhibitory IkB proteins and is released and translocated to the nucleus after phosphorylation of IkB.