Description This heterobifunctional, PEGylated crosslinker features a carboxylic acid at one end and Boc-protected amino group at the other, which can be deprotected with mildly acidic conditions. The hydrophillic PEG linker facilitates solubility in biological applications. BocNH-PEG4-acid 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 chimeras (PROTAC®molecules) fortargeted protein degradation