The NF-κB p50/p65 heterodimer is the classical member of the Relfamily of transcription factors which regulate diverse cellular functions such as immune response, cell growth, and development^,,. Other mammalian Rel family members, including theproteins p52, proto-oncoprotein c-Rel, and RelB, all have amino-terminal Rel-homology regions (RHRs)^,,,. The RHR is responsible for the dimerization, DNA binding and cytosolic localization of these proteins by virtue of complex formation with inhibitor κB proteins. Signal-induced removal of κB inhibitors allows translocation of dimers to the cell nucleus and transcriptional regulation of κB DNA-containing genes. NF-κB specifically recognizes κB DNA elements,, with a consensus sequence of 5′-GGGRNYYYCC-3′ (R is an unspecified purine; Y is an unspecified pyrimidine; and N is any nucleotide). Here we report the crystal structure at 2.9 Å resolution of the p50/p65 heterodimer bound to the κB DNA of the intronic enhancer of the immunoglobulin light-chain gene. Our structure reveals a 5-base-pair 5′ subsite for p50, and a 4-base-pair 3′ subsite for p65. This structure indicates why the p50/p65 heterodimer interface is stronger than that of either homodimer. A comparison of this structure with those of other Rel dimers reveals that both subunits adopt variable conformations in a DNA-sequence-dependent manner. Our results explain the different behaviour of the p50/p65 heterodimer with heterologous promoters.