Application： This heterobifunctional, PEGylated crosslinker features a hydroxyl group at one end and t-butyl-protected carboxylic acid at the other, which can be deprotected with acidic conditions. The hydrophillic PEG linker facilitates solubility in biological applications. Hydroxy-PEG4-t-butyl ester can be used for bioconjugation or as a building block for synthesis of small molecules, conjugates of small molecules and/or biomolecules, or other tool compounds for chemical biology and medicinal chemistry that require ligation.