m-PEG3-S-Acetyl is a PEG linker with a sulfur-acetyl end group. The sulfur acetyl group can be deprotected to generate thiol groups. After the formation of the thiol group, it is able to react with maleimides, disulfides, haloacteamides and other thiols. The hydrophilic PEG linker increases the solubility of the compound in aqueous media.