Viral vectors (ex. adeno-associated viruses, or AAV) are a common method to transfer light-sensitive opsins into the brain for experiments involving optogenetics. By packaging opsin genes fused with cell-type specific promoters into the viral vector, the opsin is expressed in cell types of interest. Promoters must be small enough to be packaged into viral vectors. In vivo, viral vectors are delivered via stereotaxic injections.