Cryptochromes,a kind of blue-light receptors,regulate a wide variety of responses such as growth,development and entrainment of circadian rhythms in diverse organisms.More interestingly,they are involved in the sensing of magnetic fields for some species.Algae,plant and animal cryptochromes possess vastly different size carboxy-terminal (C-terminal) extensions,while the C-terminal domainis hypothesized to participate in the blue-light response signaling mechanism.Here,single molecule F(o)rster Resonance Energy Transfer (smFRET) is used to study the conformation distributions of blue-light sensing algae cryptochrome (aCry).Exposure of aCry to blue-light causes a conformational change different from those of dark,while chemical reduction of the flavin induces further conformational changes in the presence and absence of blue light.