ChR2. Variants with extended open time (ChR2-XXL) produce extremely large photocurrents and are very light sensitive on the population level. Wavelength Chimeric channelrhodopsins have been developed by combining transmembrane helices from ChR1 and VChR1, leading to the development of ChRs with red spectral shifts (such as C1V1 and ReaChR). ReaChR has improved membrane trafficking and strong expression in mammalian cells, and has been used for minimally invasive, transcranial activation of brainstem motoneurons. Searches for homologous sequences in other organisms has yielded spectrally improved and stronger red-shifted channelrhodpsins (Chrimson). In combination with ChR2, these yellow/red light-sensitive channelrhodopsins allow controlling two populations of neurons independently with light pulses of different colors. A blue-shifted channelrhodopsin has been discovered in the alga Scherffelia dubia. After some engineering to improve membrane trafficking and speed, the resulting tool (CheRiff) produced large photocurrents at 460 nm excitation. It has been combined with the Genetically Encoded Calcium Indicator jRCaMP1b in an all-optical system called the OptoCaMP. Ion selectivity Most channelrhodopsins are unspecific cation channels. When expressed in neurons, they conduct mostly Na+ ions and are therefore depolarizing (excitatory). Variants with moderate to high calcium permeability have been engineered (CatCh, CapChRs). K+-specific channelrhodopsins (KCRs, WiChR) were recently discovered in various protists. When expressed in neurons, potassium-selective channelrhodopsins hyperpolarize the membrane upon illumination, preventing spike generation (inhibitory). Mutating E90 to the positively charged amino acid arginine turns channelrhodopsin from an unspecific cation channel into a chloride-conducting channel (ChloC). The selectivity for Cl- was further improved by replacing negatively charged residues in the channel pore, making the reversal potential more negative. Anion-conducting channelrhodopsins (iChloC, iC++, GtACR) inhibit neuronal spiking in cell culture and in intact animals when illuminated with blue light. Applications Channelrhodopsins can be readily expressed in excitable cells such as neurons using a variety of transfection techniques (viral transfection, electroporation, gene gun) or transgenic animals. The light-absorbing pigment retinal is present in most cells (of vertebrates) as Vitamin A, making it possible to photostimulate neurons without adding any chemical compounds. Before the discovery of channelrhodopsins, neuroscientists were limited to recording the activity of neurons in the brain and correlate this activity with behavior. This is not sufficient to prove that the recorded neural activity actually caused that behavior. Controlling networks of genetically modified cells with light, an emerging field known as Optogenetics., allows researchers now to explore the causal link between activity in a specific group of neurons and