mPEG-pALD (MW 2000) is a significant unidirectional reactive PEG derivative, featuring a reactive group that can interact with the N-terminus of amines, peptides, and proteins, forming an imine with a C=N double bond, which can be further reduced. Polyethylene glycol aldehyde or ketone derivatives can form hydrolytically cleavable acylhydrazone linkages through a reaction with hydrazine or hydrazides, suitable for reversible PEGylation.