Abstract: A novel use of Fluorescence Resonance Energy Transfer wherein a labelled protein comprising a Fluorescent energy donor label and at least one energy acceptor moiety capable of accepting energy from the donor label by Förster energy transfer is exposed to incident electromagnetic energy to excite the donor moiety and the fluorescence emission of the donor is measured. The or each energy acceptor moiety has a more and less active energy acceptor state and the level of quenching of donor fluorescence is indicative of this state. The energy acceptor moiety may be converted between its states by a redox reaction, optionally involving a partner redox protein. A novel system comprising the labelled protein, a redox partner protein, ‘a light source for imposing incident light at the excitation wavelength for the fluorescent label and a light detector capable of detecting the fluorescence emitted by the label may be used in biosensors and/or to monitor enzymatic turnover.