Chronos is a mammalian codon-optimized version of the light-gated cation channel encoded by the Stigeoclonium helveticum ShChR gene. It can be used as an optogenetic tool to activate neurons; the peak activation wavelength is green light of 500nm wavelength (FBrf0224260) and it cannot be activated by light >600nm wavelength (FBrf0237393). 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 FBrf0224260 and FBrf0237393. For a comparison of Chronos with other neuron activation tools, see FBrf0224260, FBrf0237393 and PMID:26209858.