Fig. 5a–d.

General designs of FRET-based fluorescent probes. a An intermolecular probe consists of two interacting proteins that are labeled with CFP and YFP, respectively, which interact and result in FRET. b An intramolecular probe consists of CFP and YFP fused together with a cleavable linker or protein, which can be cleaved by proteolysis and disrupt FRET. c An intramolecular probe consists of sandwiching two domains between CFP and YFP, which can interact after phosphorylation or binding to calcium, resulting in a change in FRET. d An intramolecular probe consists of CFP, YFP and a protein/domain, which permits conformational change by binding to another biomolecule, leading to a change in FRET