The ability to attach fluorescent molecules onto proteins has allowed specific localization of these proteins within living cells. The traditional approach has been to purify the protein of interest then attach a fluorescent (or another useful tag) molecule in vitro, then somehow introduce the labeled protein into the cell. Whereas this and related techniques have been very useful, there are limitations because each of these three steps is often difficult. Wouldn't it be great if we had a “lock and key” system whereby a protein of interest could be modified to be a specific “lock” and a small tsggable molecule (the “key“) could diffuse into the cell and plug specifically into the lock? Albert Griffin, Stephen Adams, and Roger Tsien have developed such a system