With the aim of elucidating the biological function of hypothetical proteins unique amongst the Actynomyces subgroup of bacteria, we have solved the crystal structure of the conserved hypothetical protein Rv1155 from Mycobacterium tuberculosis at 1.8 Å resolution. Rv1155 is a homodimer both in the crystal structure and in solution and folds into two separate domains consisting of a six-stranded anti-parallel b-barrel fold flanked by two a-helices and a helix-turn-helix domain. Both domains contribute to the formation of two deep clefts at the dimer interface. The overall fold of Rv1155 strikingly resembles that of flavin mononucleotide-binding protein and pyridoxamine 5 0-phosphate oxydase, but the architecture of the putative binding pocket is markedly different, consistent with the lack of color of Rv1155 and its inability to bind FMN. Rv1155 thus appears to belong to a group of proteins with stringent conservation of the binding cleft, having evolved towards a new binding function.