Thiol-PEG3-t-butyl ester is widely used in bioconjugation and chemical synthesis due to its reactive thiol group and polyethylene glycol (PEG) spacer. The thiol group allows for selective attachment to maleimide or other thiol-reactive groups, making it valuable for modifying proteins, peptides, and other biomolecules to improve solubility, stability, or targeting. The PEG spacer enhances biocompatibility and reduces immunogenicity, making it suitable for applications in drug delivery systems, particularly in creating antibody-drug conjugates (ADCs) or PEGylated therapeutics. Additionally, it serves as a building block in organic synthesis for constructing complex molecules or functionalized materials. The t-butyl ester group provides protection for carboxylic acids, enabling controlled deprotection in multi-step synthetic processes.