Carbocyanine dyes are a class of fluorescent dyes that are widely used in biological applications. Advantages of carbocyanine dyes include: 

• Exhibit strong fluorescence, making them very useful for a wide range of imaging techniques such as flow microscopy and flow cytometry
• Exhibit high photostability so they resist degradation and fade relatively slowly when exposed to exciting light, making them useful for long-term imaging experiments
• Low cell toxicity, so they do not affect basic physiological properties, development or survival of cells
• Do not transfer detectability from labeled to unlabeled cells 
• Can penetrate cell membranes easily 
• Diffuse laterally within cellular membranes, which is useful for labeling cells and tissues with significant intrinsic fluorescence
• Can be used for retrograde and anterograde labeling, making them useful for tracing neuronal projections