Mal-PEG12-t-butyl ester is a PEG linker containing a maleimide group and a t-butyl ester group. The hydrophilic PEG spacer enhances solubility in aqueous media. The t-butyl-protected carboxyl group can be deprotected under acidic conditions. The maleimide group reacts with thiol groups to form covalent bonds, enabling the conjugation of biomolecules with thiols.