A probe comprises: (i) a target binding site peptide which is attached to a first fluorescent polypeptide; (ii) a mimic peptide which is capable of binding to the target binding site peptide and is attached to a second fluorescent polypeptide; and (iii) a linker which connects the two fluorescent polypeptides and which allows the distance between said fluorescent polypeptides to vary, said fluorescent polypeptides being such as to display fluorescence resonance energy transfer (FRET) between them.
