Aion is an engineered bistable anion-selective channelrhodopsin that can be used as an optogenetic tool to inhibit neuron activity; the peak activation wavelength is 460nm with 20ms pulsed activation. Aion was generated by introducing the D156C amino acid substitution into Phobos(CA); this greatly slows down the closing kinetics, so that Aion shows a long-lasting open state with a spontaneous closing time constant close to 15 minutes. Aion can be switched between the open and closed state with millisecond precision using blue (460nm) and orange light (595nm) respectively, allowing reversible silencing of neurons (adapted from FBrf0253932).