Biliverdin-IXareductase fromSynechocystisPCC6803 (sBVR-A) is a stable dimer and this behaviour is observed under a range of conditions. This is in contrast to all other forms of BVR-A, which have been reported to behave as monomers, and places sBVR-A in the dihydrodiol dehydrogenase⁄N-ter-minally truncated glucose–fructose oxidoreductase structural family of dimers.