Single-molecule fluorescent spectroscopy can provide unique perspectives which complement conventional bulk biochemical approaches.We use single-molecule techniques to illuminate the details of the working mechanism of signaling proteins at different stages of a signalling transduction.Light-sensitive protein cryptochrome plays many different rolls in eukaryotic cells.It is a kind of signalling proteins that is known to retrain circadian clock and involves in the magnetic sensing capabilities of animals.Using smFRET (single-molecule Foster-type resonance energy transfer),we reveal how blue light would initiate in a conformational change in an algae cryptochrome under different redox states and this conformational change is believed to be functionally important.While conformational changes are usually speculated as the cause of activation of signalling function in various receptor proteins,smFRET can afford direct evidence to these hypotheses.