ATTO 647 belongs to a new generation of fluorescent labels for the red spectral region. Characteristic features of the label are strong absorption, high fluorescence quantum yield, high photo-stability, and good water solubility. ATTO 647 is a zwitterionic dye with a net electrical charge of zero. The dye is stable at physiological pH-values and in buffers up to pH 8. However, it slowly degrades at higher pH. The dye is very hydrophilic. The fluorescence is excited most efficiently in the range 615 - 660 nm. A suitable excitation source for ATTO 647 is, next to the He:Ne laser (633nm), the 647 nm line of the Krypton-Ion laser or a diode-laser emitting at 650 nm.