HOOK™-NHS-SS-Biotin reacts with protein primary amines.  Amines, lysine ε-amines and N-terminal α-amines, are the most abundant group in protein molecules and represent the most common target for biotinylation. For example, BSA contains 59 primary amines, of which up to 35 are available on the surface of the molecules and can be reacted with amine reactive esters.