Nuclear hormone receptors are ligand-regulated transcription factors that function in metazoan homeostasis and differentiation. We report here that DNA recognition by at least one class of nuclear receptors, the retinoid X receptors (RXRs), can occur through an unanticipated mechanism involving the cooperative formation of protein tetramers and other high-order oligomers. Formation of these oligomeric complexes enables RXRs alpha and gamma, but not beta, to efficiently regulate transcription through response elements poorly recognized by RXR dimers. Thus, oligomer formation plays an important role in determining the specificity of DNA recognition by nuclear receptors and contributes to isoform-dependent differences in gene regulation.