Functional complementation screens can identify known or novel proteins with important intracellular activities. We have isolated human enhancer of filamentation 2 (HEF2) in a screen to find human genes that promote pseudohyphal growth in budding yeast. HEF2 is identical to enhancer of rudimentary homolog (ERH), a highly conserved protein of 104 amino acids. In silico protein-interaction mapping implies that HEF2/ERH interacts with transcription factors, cell-cycle regulators, and other proteins shown to enhance filamentous growth in S. cerevisiae, suggesting a context for studies of HEF2/ERH function. To provide a mechanistic basis to study of HEF2/ERH, we have determined the crystal structure of HEF2/ERH at 1.55 Å. The crystal asymmetric unit contains a HEF2/ERH monomer. The two monomers of the physiological dimer are related by the y, x, –z crystal symmetric operation. The HEF2/ERH structure is characterized by a novel α + β fold, a four-strand antiparallel β-sheet with three α-helixes on one side of the sheet. The β-sheets from the two monomers together constitute a pseudo-β-barrel, and form the center of the functional HEF2/ERH dimer, with a cavity channel at the dimer interface. Docking of this structure to the HEF2/ERH partner protein DCOH/PCD suggests that HEF2/ERH may regulate the oligomeric state of this protein. These data suggest that HEF2/ERH may be an important transcription regulator that also functions in the control of cell-cycle progression. Proteins 2007. © 2007 Wiley-Liss, Inc.