N-Boc-N'-(PEG1-t-butyl ester)-L-Lysine-NHS ester is the amino acid lysine with a Boc-protected α-amine, an amido-PEG1-t-butyl ester on its ε-amine, and an NHS ester on its C-terminus. The NHS can be easily displaced by amines to form an amide, while the Boc and t-butyl ester can be deprotected to perform further conjugation.