Currently, there is no technique available that enables specific and inducible genetic manipulation of astrocytes in the brain. Here, the authors identified a marker that enabled selective targeting of astrocytes in mice using the Cre–LoxP system. They combined this with expression of a genetically encoded calcium indicator and used the system to detect intracellular calcium signals in astrocyte processes during a startle response, demonstrating the usefulness of this approach for the study of astrocytes and behaviour.