Red-shifted opsin

Opsins engineered to absorb longer (orange, red, to near-infrared) wavelengths rather than the blue light that drives ChR2. Because tissue scatters and absorbs red light far less than blue, red-shifted actuators (ChrimsonR, ReaChR, bReaChES, ChRmine) penetrate deeper and cause less phototoxicity; equally important, their spectral separation from green/yellow calcium and voltage indicators is what makes crosstalk-limited all-optical experiments feasible. ChRmine in particular combines a red shift with very large photocurrent, enabling transcranial or minimally invasive stimulation of deep structures in rodents.

The trade-off is that red-shifted opsins usually retain a broad activation spectrum with residual blue sensitivity — a hidden source of crosstalk when paired with a blue-excited indicator — and some (ChRmine) have slow kinetics that cap temporal precision. The first reported partial restoration of visual function in a blind person used a red-shifted opsin (ChrimsonR) expressed in retinal ganglion cells together with amber-light stimulating goggles: a landmark, but a single-subject and only partial result.