Biotin:avidin second antibody signal systems involve the conjugation of biotin to a secondary antibody. Once the secondary antibody binds to a target primary antibody in an immunochemical assay, the signal is developed by allowing avidin which has been conjugated to an enzyme or signal molecule to bind to the antibody associated biotin molecule. Water soluble biotinylation reagent. Typically coupled to primary amine in the pH range 6.5-8.5. Fluorogenic reagent for specific C-terminal labeling of peptides and proteins by carboxypeptidase Y catalyzed transpeptidation reactions.