eNpHR3.0 is an engineered light-gated chloride pump that can be used as an optogenetic tool to induce membrane hyperpolarization and inhibit neuron activity. eNpHR3.0 was generated by adding a C-terminal Golgi trafficking signal (from Kir2.1) to eNpHR2.0. This change improves cell-membrane expression, particularly in neuronal processes. The trafficking signal is upstream of the ER export signal present in eNpHR2.0, and any fluorescent protein added to a particular transgene or modified endogenous locus as a tag to facilitate detection of the eNpHR3.0 protein is typically inserted in between the Golgi trafficking and ER export signals (PMID:20303157). 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 a comparison of eNpHR3.0 and other neuron inhibition tools, see PMID:20303157, PMID:21415127, PMID:21745635 and PMID:22179551.