The isomer-free, amine-reactive Alexa Fluor 680 carboxylic acid, succinimidyl ester and its conjugates are spectrally similar to Cy5.5 dye. The spectral separation from other commonly used red dyes, such as Texas Red dye, Alexa Fluor 594 dye and Alexa Fluor 660 dye, makes the Alexa Fluor 680 dye a perfect choice for three- and four-color labeling. A 1 mg unit size (A-20008) is also available. Fluorescence of this long-wavelength Alexa Fluor dye is not visible to the human eye but is readily detected by most imaging systems.