HOOK™-PEG2-Iodoacetyl-Biotin reacts with protein free sulhydryl groups. Sulfhydryl reactive reagents are more specific and react only with free sulfhydryl residues (-SH or thiol groups). The side chain of the amino acid cysteine is the most common source of free sulfhydryl groups. If free sulfhydryl resides are not available, they can be generated by either the reduction of disulfides (-S-S-) with reducing agents such as mercaptoethylamine, or by modifying lysine E-amines with Traut's reagent or SATA.