SUMMARY
In this article, we have focused on the diversity of the microbial opsin genes and the structure–function properties of the corresponding proteins. Although much has been learned in applying the use of microbial opsins to neuroscience, technological challenges lie ahead, including development of variants with further red-shifted absorption or altered ion selectivity, as well as more complex activation schemes with multiple optogenetic proteins. Biochemical tools are also now available for specific GPCR pathways (Airan et al. 2009), and increasingly complex avenues of control and spectral separation will aid in the design of more refined experiments that do justice to the combinatorial complexity of the mammalian brain. Further rapid expansion of the optogenetic toolkit now will be a natural consequence not only of ever-increasing genomic and metagenomic resources, but also of molecular engineering spurred by increased understanding of the structure–function relationships of these ancient and powerful molecular machines.