The weakly fluorescent DAF-2 is a sensitive fluorogenic indicator commonly used for detecting nitric oxide (NO). It reacts with NO in the presence of oxygen to yield the highly fluorescent triazolofluorescein. Its fluorescence can be monitored using the standard FITC filter set. At neutral pH the detection limit is 2-5 nM for NO. It might be used for real-time NO detection, but the optimal DAF-2 concentration must be experimentally determined for a cellular NO detection. For example, a low DAF-2 concentration (e.g., 0.1 uM) was used for the real-time detection of endothelial NO formation by subtracting the DAF-2 auto-fluorescence from the measured total fluorescence.