erh (enhancer of rudimentary homolog) is a ubiquitously expressed transcriptional coregulator that is highly conserved among eukaryotes, from humans to plants to protozoa. Functions attributed to erh include enhancement of pyrimidine biosynthesis, a role in cell cycle regulation, and repression of the tissue-specific transcription factor HNF-1 (hepatocyte nuclear factor-1) through binding the coactivator DCoH (dimerization cofactor of HNF1). No homologous sequences, other than erh orthologs, have been identified, and little is known about the interactions of erh. To further elucidate its function, we determined the crystal structure of erh to 2.0 A resolution. The erh structure is a novel alpha + beta fold consisting of a four-stranded antiparallel beta sheet with three amphipathic alpha helices situated on one face of the beta sheet. Structure-based searches of the Protein Data Bank, like sequence-based searches, failed to identify paralogs. We present structural and biochemical evidence that erh functions as a dimer. The dimer interface consists of a beta sandwich composed of the beta sheet from each monomer. Many of the surface residues of erh are conserved, including patches of hydrophobic and charged residues, suggesting protein-protein interaction interfaces. Two putative CKII phosphorylation sites are highly ordered in the structure and are predicted to disrupt dimerization and protein-protein interactions.