ChR2(C128T) 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 a C128T amino acid substitution. This mutation slows off kinetics, resulting in ChR2(C128T) behaving as a bistable channelrhodopsin; it is activated by a brief pulse of blue light (peak wavelength 470nm) to produce a prolonged photocurrent with a longer duration than the initiating pulse (time constant of spontaneous decay is approximately 2 seconds), resulting in a stable step in membrane potential, and it can be inactivated by a second light pulse of longer wavelength (PMID:19079251). 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:19079251. For a comparison of ChR2(C128T) and other neuron activation tools, see PMID:20621963 and PMID:21745635.