mPEG-pALD (MW 1000) is a crucial mono-functional PEG derivative. It contains reactive groups capable of reacting with the N-terminus of amines, peptides, and proteins to form imines with C=N double bonds, which can be further reduced. Polyethylene glycol aldehydes or ketone derivatives can form hydrolysable acylhydrazone bonds through reactions with hydrazine or hydrazides, enabling reversible PEGylation.