CBS domains are small protein motifs consisting of a three-stranded beta-sheet and two alpha-helices that are present in proteins of all kingdoms of life and in proteins with completely different functions. Several genetic diseases in humans have been associated with mutations in their sequence, which has made them promising targets for rational drug design. The C-terminal domain of the Methanococcus jannaschii protein MJ0100 includes a CBS-domain pair and has been overexpressed, purified and crystallized. Crystals of selenomethionine-substituted (SeMet) protein were also grown. The space group of both the native and SeMet crystals was determined to be orthorhombic P2(1)2(1)2(1), with unit-cell parameters a = 80.9, b = 119.5, c = 173.3 A. Preliminary analysis of the X-ray data indicated that there were eight molecules per asymmetric unit in both cases.