Abstract: A method of detection of an analyte in which (i) a protein comprising a moiety capable of binding the analyte and a fluorescent label is contacted with a medium suspected of containing the analyte; (ii) the analyte, if present, binds to the moiety; (iii) the protein is subjected to incident radiation to excite the protein and induce intrinsic emission therefrom; whereby the intrinsic emission from the protein is converted through Fluorescence Resonance Energy Transfer (FRET) into emission from the fluorescent label and the amount of said FRET is affected by the binding of the analyte to the moiety; and, (iv) the emission from the fluorescent label is measured; whereby the level of emission from the fluorescent label is indicative of the presence of the analyte, and wherein the protein undergoes no substantial conformational charge during the method. A kit for carrying out the method of the invention and a protein are also provided.