Fast Blue BB salt is used as a general substrate for β-glycosidase. Two moles of 6-bromo-2-napthol-β-d-glucopyranoside are cleaved by β-glucosidases into 2 moles of d-glucose and 2 moles of 6-bromo-2-napthol. The 2 moles of 6-bromo-2-napthol formed are coupled with 1 mole of Fast Blue BB salt [4-benzoylamino-2,5-diethoxybenzenediazonium chloride hemi (zinc chloride) salt], resulting in 2 moles of the corresponding dye of 6-bromo-2-naphthol (reddish color). It is a commonly used stain to observe myelin under light microscopy.
