Atto 647N is a superior red-emitting label with high molecular absorption (150.000) and quantum yield (0.65) as well as sufficient stoke′s shift. Atto 647N is characterized by a high thermal and photostability. Absorption and fluorescence are independent of pH, at least in the most relevant range of pH 4 to 11. The azide modification is suitable for reactions with alkyne groups (Huisgen reaction - "Click Chemistry").