The v- and c-Rel oncoproteins bind to oligonucleotides containing kappa B motifs, form heterodimers with other members of the Rel family, and modulate expression of genes linked to kappa B motifs. Here, we report that the RxxRxRxxC motif conserved in all Rel/kappa B family proteins is absolutely required for v-Rel protein-DNA contact and its resulting transforming activity. We also demonstrate that serine substitution of the cysteine residue conserved within this motif enables v-Rel to escape redox control, thereby promoting overall DNA binding. These mutant proteins retained the ability to competitively inhibit kappa B-mediated transcriptional activation of the human immunodeficiency virus long terminal repeat but failed to efficiently transform chicken lymphoid cells both in vitro and in vivo. Our data indicate that reduction of the conserved cysteine residue in the RxxRxRxxC motif may be required for optimal DNA-protein interactions. These results provide direct biochemical evidence that the DNA-binding activity of v-Rel is subject to redox control and that the conserved cysteine residue in the RxxRxRxxC motif is critical for this regulation. These studies suggest that the DNA-binding, transcriptional, and biological activities of Rel family proteins may also be subject to redox control in vivo.