FRET between two fluorescent proteins linked by the calmodulin binding domain used as a Ca2+ sensor named Cameleon.

References:

A Miyawaki, J Llopis, R Heim, JM McCaffrey, JA Adams, M Ikura, RY Tsien: Fluorescent indicators for Ca2+ based on green fluorescent proteins and calmodulin. Nature 388 (1997) 882-87.

A Miyawaki, O Griesbeck, R Heim, RY Tsien: Dynamic and quantitative Ca2+ measurements using improved cameleons. Proc. Nat. Acad. Science 96 (1999) 2135-40.