Conjugates prepared with the thiol-reactive Alexa Fluor® 633 C₅ maleimide are efficiently excited with the 633 nm spectral line of the He-Ne laser and the 635 nm spectra line of the red diode laser. Far-red-fluorescent dyes are among the most sought-after labels for fluorescence imaging because their spectra are well beyond the range of most sample autofluorescence.