Looking back at the development of PEGylation, it is apparent that, until recently, the preponderance of PEGylation reagents primarily targeted the amino groups on side chains of lysine residues in proteins, which is considered random PEGylation. Lysines are relatively numerous, polar amino acid residues typically found on the surface of proteins, making them susceptible to chemical interactions with PEG reagents. As a result, these reactions proceed rapidly and produce complex mixes of conjugates that vary in the quantity and location of the attached PEG chains (Cheng etal. 2019). These reactions are not conned to the amino groups of lysine residues; they also somewhat react with certain other protein nucleophiles, such as the side chains of serine, threonine, and tyrosine cysteine residues, the N-terminal amino groups and the imidazolyl nitrogens of histidine residues. Although the pH of the medium can inuence the reaction to some degree, these conjugation processes typically result in complex PEGylation mixes (Maiser et al. 2014). Also, rst­generation high molecular weight, pure monofunctional PEG reagents are challeng­ing to construct and ineffective for protein conjugation because they have a 15% maximum diol concentration (Zhang etal. 2007).