Credit: Andre Berndt, Soo Yeun Lee, Charu Ramakrishnan, and Karl Deisseroth.
Channelrhodopsins before (upper left) and after (lower right) molecular engineering, shown superimposed over an image of a mammalian neuron. In the upper left opsin, the red color shows negative charges spanning the opsin that facilitated the flow of positive (stimulatory) ions through the channel into neurons. In the newly engineered channels (lower right), those negative charges have been changed to positive (blue), allowing the negatively charged inhibitory chloride ions to flow through.