N-Boc-N'-(PEG1-t-butyl ester)-L-Lysine-OH is the amino acid lysine with a Boc-protected α-amine and an amido-PEG1-t-butyl ester on its ε-amine. The carboxylic acid can be reacted with alcohols or amides, while the Boc and a t-butyl ester which can be later deprotected to perform further conjugation.