To evaluate whether AAV9-Retro can achieve high-efficiency retrograde labelling input neurons, 300 nl of the rAAV9-Retro-CaMKIIa-EGFP virus was infused into the VTA, and then local infection and the cortical region projecting to the VTA were imaged by confocal microscopy at 21 days post-injection (DPI). This evidence concerns the gene CAMK2A and infection.