Fig. 2.

Overview of polysynaptic versus monosynaptic tracing with H129-derived tracers. A Schematic of the mechanism of polysynaptic tracing using H129-G4. B Flowchart and timeline for polysynaptic tracing using H129-G4. C Schematic of the mechanism of monosynaptic tracing using H129-dTK. H129-dTK virus can infect neurons normally by itself, but cannot replicate in neurons due to TK deficiency. TK expression by helper AAV-TK assists with H129-dTK replication, and offspring viruses transmit to the 2nd-order neurons and label them with the fluorescent protein. D Flowchart and timeline for monosynaptic tracing using H129-dTK tracers.