Tetrafluorophenyl (TFP) esters are an improvement over the succinimidyl ester (SE or NHS-ester) chemistry typically used to attach fluorophores or haptens to the primary amines of biomolecules. Both reactive chemistries produce the same strong amide bond between the dye or hapten and the compound of interest, but TFP esters are less susceptible to spontaneous hydrolysis during conjugation reactions. Alexa Fluor TFP esters are stable for several hours at the basic pH typically used for reactions – far outlasting succinimidyl esters.