A new optogenetic tool, a protein that can be controlled by light, has been characterized by researchers at Ruhr-Universität Bochum (RUB). They used an opsin—a protein that occurs in the brain and eyes—from zebrafish and introduced it into the brain of mice. Unlike other optogenetic tools, this opsin is not switched on but rather switched off by light. Experiments also showed that the tool could be suitable for investigating changes in the brain that are responsible for the development of epilepsy.