The nuclear transcription factor NF-κB is a dimer composed of Rel protein family members. In mammals, these proteins include P50(NF-κB1), P52(NF-κB2), P65(RelA), RelB, C-REL, P105, and P100. They all contain a conserved Rel homologous domain (RHD) consisting of 300 amino acids at the N-terminus, which has multiple functions including DNA binding, homologous or heterologous dimerization, nuclear localization, and interaction with members of the IkB family.