ChR2-XXM is a light-gated cation channel that can be used as an optogenetic tool to activate neurons. It is an engineered derivative of ChR2, containing the D156H amino acid substitution relative to ChR2; this mutation results in exceptionally high protein expression in addition to a medium open state (FBrf0236304). 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 FBrf0236304.