mPEG-pALD (MW 20000) is a significant mono-reactive PEG derivative, equipped with reactive groups that can react with the N-terminus of amines, peptides, and proteins to form an imine containing a C=N double bond, which can be further reduced. Polyethylene glycol aldehyde or ketone derivatives can form hydrolysable acylhydrazone bonds through reactions with hydrazine or hydrazide, making them suitable for reversible PEGylation.