Chrimson is a mammalian codon-optimized version of the light-gated cation channel encoded by the Chlamydomonas noctigama CnChR1 gene. It can be used as an optogenetic tool to activate neurons; the peak activation wavelength is approximately 590nm. 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.