MSD TAG NHS-Ester is an N-hydroxysuccinimide ester which readily couples to primary amine groups of proteins. MSD TAG conjugated proteins are used as detection reagents in MSD immunoassays. The conjugates are stable, may be used at low concentrations, have low non-specific binding, and result in highly sensitive detection. The simple conjugation procedure of MSD TAG NHS-Ester make it the product of choice for molecules that contain primary amines (e.g., lysine in proteins). MSD TAG NHS-Ester can be used to tag proteins in organic solvents and contains minimal electrical charge.