Application： This heterobifunctional, PEGylated crosslinker features an amino group at one end and t-butyl-protected carboxyl group at the other, which can be deprotected with acidic conditions. The hydrophillic PEG linker facilitates solubility in biological applications. Amino-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. Examples of applications include its synthetic incorporation into antibody-drug conjugates or proteolysis-targeting chimera