Chang, H. et al. Proc. Natl. Acad. Sci. USA advance online publication (28 February 2012).

The advent of super-resolution microscopy techniques that rely on fluorescent molecules that can be switched on and off or between two different colors, so as to localize subsets of tagged proteins and build up an image, has stimulated the development of new variants of photoactivatable fluorescent protein tags. Monomeric (m)Eos2, a protein that irreversibly switches from green to red when illuminated with violet light is one of the more popular ones. Chang et al. used targeted mutagenesis of mEos2 to eliminate the red form and make the green form reversibly switch off and on when illuminated with alternating violet and cyan light. This resulted in six reversibly photoswitchable monomeric green fluorescent proteins with different photophysical properties. These genetically encoded protein tags join a rapidly expanding toolbox of photoactivatable fluorescent proteins. A variety of choices is good, but researchers may now need help choosing among the available tools.