To gain a clearer understanding of how brain circuits work, it is important to examine and manipulate the function of specific brain nuclei and neuron types. Recombinant viral technology facilitates targeted transgene expression in specific neuronal populations. In the past decades, the viral vectors allow the mapping, observation, and manipulation of neuronal circuits in experimental models. For example, the CVS-N2c strain and SADB19-base vectors have been used for the monosynaptic tracing of neuronal connections. What's more, the combination of cell type-specific viral vectors with trans-neuronal vectors can be used to dissect input-output circuits. Learn more: Viral Vectors Neural Circuit Mapping.