Phobos is an engineered blue-shifted anion-selective channelrhodopsin that can be used as an optogenetic tool to inhibit neuron activity; the peak activation wavelength is 467nm with 10ms pulsed activation. Phobos was generated by adding the amino acid substitutions T159G and G163A to the iC++ channelrhodopsin (FBrf0237123). The formation of a functional channel requires the presence of a covalently linked all-trans-retinal chromophore which can be provided exogenously if necessary via the culture medium or diet (this cofactor is present endogenously in some intact vertebrate systems).