The eukaryotic transcription factors NF-kappaB P50 and NF-kappaB P52 are closely related members of the Rel family. Growth of diffraction-quality NF-kappaB P52:DNA co-crystals crucially depended on (a) extensive screens for the DNA fragment of optimal length and (b) engineering of the protein based on the two known NF-kappaB P50:DNA co-crystal structures [Müller et al. (1995) Nature 373, 311-317; Ghosh et al. (1995) Nature 373, 303-310]; namely, deletion of 12 C-terminal amino acid residues. These residues are part of the Rel homology region and comprise the nuclear localization signal. The approach might be of general use for the crystallization of other Rel protein: DNA complexes and in our case yielded co-crystals which diffract beyond 2.0 A resolution.