ChIEF is a light-gated cation channel that can be used as an optogenetic tool to activate neurons; it is optimally excited by blue light of approximately 450nm wavelength. ChIEF is an artificial channelrhodopsin that combines sequence from the ChR1 and ChR2 genes of Chlamydomonas reinhardtii; it consists of the extracellular N-terminus and transmembrane domains A-E of ChR1 and transmembrane domains F-G of ChR2, with a I170V point mutation in transmembrane domain C (PMID:19254539). 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). For detailed kinetic and spectral properties, see PMID:19254539 and PMID:22179551. For a comparison of ChIEF and other neuron activation tools, see PMID:20621963 and PMID:22179551.