Two years after Betzig’s proposal, Moerner was studying the light emission properties of single molecules of mutants of the green fluorescent protein (GFP) obtained from Roger Tsien’s laboratory. The studies (Dickson et al., 1997) were performed at ambient temperature with the protein embedded in an aerated aqueous gel. Unexpectedly, when excited at 488 nm the protein was seen to undergo several cycles of intermittent fluorescence emission: the GFP molecules displayed “blinking” behaviour. After several blinking cycles, the molecules went into a stable dark state. Amazingly, from this dark state they could be re-activated by irradiation at 405 nm. These results demonstrated for the first time that it was possible to optically guide fluorescent proteins between active and inactive states by the sophisticated use of the inherent photochemistry of the protein (Fig. 7).