ChR2-XXL is a light-gated cation channel that can be used as an optogenetic tool to activate neurons; the peak activation wavelength is approximately 477nm (PMID:22713581). It is an engineered derivative of ChR2, containing the D156C amino acid substitution relative to ChR2; this mutation results in exceptionally high protein expression in addition to a long open state (PMID:22713581, FBrf0226366). 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).