This image shows expression of an optimized hybrid voltage sensor (hVOS) probe in the dentate gyrus of an Ai35 hVOS:: FOS mouse, viewed with 2-photon microscopy. Neurons expressing the voltage probe were activated by exposing the mouse to a novel environment. The hVOS probe enabled fluorescence imaging of voltage changes in these neurons. For more information, see the article by Bayguinov et al. (pages 9305–9319).