Here we will identify nonhuman primate (NHP) neuron types and build an extensive toolbox of vectors for circuit- based neuroscience studies. NHPs share substantial neuroanatomical, genetic, and behavioral homology with humans, and therefore they are indispensable for investigating the neural circuit basis of cognition and devising therapies to treat neurological and psychiatric disorders. Despite the importance of NHPs, we lack the tools to analyze and manipulate complex circuits in the primate brain. This lack severely limits the use of genetically- coded neuroscience tools to examine circuit specific functions and hinders development of targeted gene therapeutics. Current methods for achieving transgenesis in small model species, such as the creation of genetically modified strains, are prohibitively expensive in NHP and not applicable to human disease. AAVs are the leading alternative to germline modification and selective breeding. AAVs infect adult neurons, confer stable transgene expression, and have proven safe in gene therapy clinical trials. AAVs do not have natural cell-types specific properties, but when altered or combined with cell type specific regulatory sequences (enhancers/promoters) they have been able to achieve cell type-specific transgenesis. This has made possible, for example, our previous optogenetic investigation of midbrain dopamine neurons for learning and decision making. However, before AAV-mediated gene delivery can be generalized to circuits across the brain and for multiple behavioral functions, we must create currently lacking vectors and promoters that permit efficient and specific gene delivery to all required cell types. Here, we will combine single-cell RNA-Seq (scRNA-Seq) with high-throughput screening of engineered adeno-associated viruses (AAVs) to create a complete toolbox of viral vectors and promoters enabling minimally invasive monitoring and manipulation of neurons in NHP brain. We have devised a transdisciplinary approach to classify individual neurons according to their gene expression profile and simultaneously screen for adeno-associated virus (AAV) vectors (capsids and regulatory sequences) capable of specific and efficient transgene delivery to classified neurons. First, we will synthesize massive libraries of mutated AAV vectors and synthetic promoters, in which each variant is paired with a unique DNA barcode. We will then scRNA-Seq to capture the transcriptome for each cell and quantify the AAV and promoter- specific barcodes in every cell’s expression profile. Preliminary experiments in Rhesus monkeys have fully validated and demonstrated the promise of this innovative approach. The outcomes of our Specific Aims will include (1) an inventory of cell types in the retina, prefrontal cortex, primary motor cortex, and striatum, (2) cell type-specific AAVs and promoters targeting all defined cell types, (3) AAVs with broad tropisms, (4) a publicly available dataset of transcription profiles for mi...