In summary, the ER protein, which is highly conserved among organisms as diverse as vertebrates, invertebrates, and plants, has been implicated as functioning in pyrimidine biosynthesis and the cell cycle. In this study, ER was found to behave as a dimer in solution, and the solution structure of the ER monomer was determined by heteronuclear NMR spectroscopy. The ER monomer consists of a four-stranded antiparallel b-sheet, with a strand order of β2β1β3β4, and three a-helices (α1, α2, and α3) packed against one side of the sheet, with an overall topology of β1β2α1α2β3β4α3. A structural homology search revealed that ER forms a novel fold. These structural features of ER will shed light on its functional mechanism at the molecular level. The coordinates have been deposited in the Pro tein Data Bank (accession code 1WWQ).