We have been adapting genetically-encoded reagents from nature and engineering them in order to sensitize cellular processes to being controlled by light. One of our areas of innovation is in creating molecular tools that, when genetically targeted to specific neurons, allow them to be activated by brief pulses of light. In this way, the sufficiency of a neural pathway, cell type, or brain region in generating a given behavior, neural computation, or pathology can be assessed. Furthermore, these ultraprecise tools may empower a new generation of optical control prosthetics. The reagent channelrhodopsin-2 (ChR2), for example, depolarizes cells in which it is expressed, in response to pulses of blue light.