BIS NHS (PEG)n Reagents are homobifunctional crosslinkers for covalent conjugation between amine-containing molecules. Crosslinkers having polyethylene glycol (PEG) spacers are convenient and useful alternatives to those with purely hydrocarbon spacer arms. PEG spacers improve water-solubility of reagent and conjugate, reduce the tendency of conjugates to aggregate upon storage, and decrease immunogenic response to the spacer itself. By contrast to typical PEG reagents that contain heterogeneous mixtures of different PEG chain lengths, Pierce PEG reagents are homogeneous compounds of defined molecular weight and spacer arm length, providing greater precision in optimization and characterization of crosslinking applications. Homobifunctional crosslinkers containing N-hydroxysuccinimide (NHS) esters are often used for low-resolution 3-D studies of protein structure and protein interaction analysis.