SPDP-PEG6-NHS ester is a PEG derivative with SPDP and NHS ester moieties. The SPDP is an amine and thiol reactive crosslinker. It is also membrane permeable, allowing it to participate in intracellular crosslinking reactions. The NHS ester is amine reactive and forms stable amide bonds. The hydrophilic PEG linker increases the solubility of the compound in aqueous media.