Aminooxy-PEG-methyl ester is a compound with a functional group that contains an aminooxy (ONH2) group and a polyethylene glycol (PEG) chain. This functional group has been widely used in pharmaceutical research and development due to its ability to selectively label and modify biomolecules. Specifically, aminooxy-PEG-methyl ester can react with carbonyl groups of biomolecules such as aldehydes and ketones to form a stable oxime bond. This reaction has been used for drug conjugation, protein labeling, and bioconjugation. Furthermore, the PEG chain in aminooxy-PEG-methyl ester can improve the solubility and stability of the resulting conjugates, making it a valuable tool in drug delivery and therapeutic applications.