Features:HSV-ΔTK-hUbC-tdTomato is obtained by knocking out the thymidine kinase (TK) gene based on HSV-tdTomato (HSV1 strain H129 genome integrating a tdTomato gene). Due to the knockout of the TK gene, its ability to spread across synapses in vivo is limited. In the presence of a TK-expressing helper AAVs, HSV-ΔTK-hUbC-tdTomato can spread from the initial neurons to the postsynaptic neurons, thereby labeling the immediate downstream neurons directly innervated by a given brain area or specific type of neurons.