Amino-PEG12-t-butyl ester is a aquepous soluble PEG linker containing an amino group (NH2) with a t-butyl protected carboxyl grou (Boc). The amino group is reactive with carboxylic acids, activated NHS esters, carbonyls (ketone, aldehyde) etc. The t-butyl protected carboxyl group can be deprotected under acidic conditions.