N-(Azido-PEG4)-biocytin is a PEG-based linker used in the synthesis of PROTACs. It is a biotin PEG analog containing a carboxylic group and an azide group. Reaction of carboxylic with primary amino (-NH2) forms stable, irreversible (covalent) amide bonds. The azide group can react with alkyne moiety in Cu(I)-catalyzed Click Chemistry reaction or DBCO moiety in copper-free Click Chemistry reaction to form a stable triazole linkage. The hydrophilic PEG spacer increases solubility in aqueous media and increases membrane impermeability of the molecules conjugated to the biotin compound.