Description This heterobifunctional, PEGylated crosslinker 17-Azido-3,6,9,12,15-pentaoxaheptadecanoic acid features a carboxyl group at one end and azide at the other. The hydrophillic PEG linker facilitates solubility in biological applications. This azido-PEG5-acid linker 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.