mPEG-pALD (MW 3400) is a significant mono-reactive PEG derivative featuring reactive groups that can bond with the N-terminus of amines, peptides, and proteins to create imines with C=N double bonds, which can be further reduced. Polyethylene glycol aldehyde or ketone derivatives can form hydrolytically cleavable acylhydrazone bonds through reaction with hydrazine, enabling reversible pegylation.