Aurora is an engineered anion-selective channelrhodopsin that can be used as an optogenetic tool to inhibit neuron activity; the peak activation wavelength is 517nm with 10ms pulsed activation, while longer light pulses (500ms) with 10-fold increased photon irradiance show two activation peaks, at 419nm and 585nm. Aurora was derived from ReaChR by adding mutations and an N-terminal sequence from the iC++ channelrhodopsin, resulting in Aurora containing sequence from four different naturally occurring channelrhodospin genes; VChR1 and VChR2 from Volvox carteri, and ChR1 and ChR2 from Chlamydomonas reinhardtii (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).