The crystal structure of the T110078 protein from Thermosynechococcus elongatus BP-1, which contains a BLUF (a sensor of blue-light using FAD) domain bound to FAD was determined at 2 Å resolution. The overall structure of BLUF domain consists of a five-stranded mixed-β-sheet with two α-helices running parallel with it. The isoalloxazine ring of FAD is accommodated in a pocket formed by several highly conserved amino acid residues in the BLUF domain.