cis-hexahydro-2-oxo-1H-thieno[ 3,4-d]imidazoline-4-valeric acid; the D-isomer component of the vitamin B2 complexoccurring in or required by most organisms and inactivated by avidin; participates in biological carboxylations. It is a smallmolecule with a high affinity for avidin that can be readily coupled to a previously labeled antibody in order to allowvisualization by enzymatic or histochemical means. Syn: coenzyme R, vitamin H, Wfactor.