(2-Pyridyldithio)-PEG4 acid is a four-unit cleavable polyethylene glycol (PEG) linker specifically designed for the synthesis of antibody-drug conjugates (ADCs)[1]. It serves as a crucial component in the conjugation of antibodies and drugs, enabling targeted drug delivery and enhanced therapeutic efficacy. This linker possesses a (2-pyridyldithio) group at one end, facilitating the attachment of the linker to the specific site on the antibody molecule. The PEG4 chain provides the necessary flexibility and biocompatibility for optimal drug release while maintaining stability throughout the circulation in the body. Ultimately, (2-Pyridyldithio)-PEG4 acid is instrumental in the development and advancement of ADCs, a promising approach in cancer therapy.