ChETA(E123T.H134R) 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 E123T and H134R amino acid substitutions relative to ChR2 (PMID:20081849). 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 a comparison of ChETA(E123T.H134R) with other neuron activation tools, see PMID:20621963, PMID:21415127 and PMID:22179551.