TABLE 1.

Properties of commonly used viral vectors in neural circuit tracing.

Type			Virus	Genome size	Vector capacity	Cytotoxicity	Expression efficiency		Transport characteristics
Anterograde	Non-transsynaptic		AAV	∼4.7 kb	∼4.7 kb	Low	Depends on different serotypes	Disadvantage:	Potential retrograde transport
	Trans-synaptic	Trans-monosynaptic	AAV1 with high titer	∼4.7 kb	∼4.7 kb	Low	Low trans-synaptic efficiency	Disadvantage:	Potential retrograde transport
			HSV1-H129-dTK	∼150 kb	∼50 kb	High	Low (due to TK deficiency)	Disadvantage:	Potential retrograde transport
		Trans-multisynaptic	HSV1-H129	∼150 kb	∼50 kb	High	High	Disadvantage:	Potential retrograde transport
			VSV	∼11 kb	∼4 kb	High	High	Disadvantage/advantage:	Bi-directional transport, but pseudotyped VSV with RV-G shows complete retrograde transport
Retrograde	Non-transsynaptic		AAV-retro	∼4.7 kb	∼4.7 kb	Low	Limited subcortical infection	Advantage:	Efficient axon terminal absorption
			CAV-2	∼31 kb	∼30 kb	Moderate	Moderate	Advantage:	Preferentially transduces neuronal axon terminals
			RVdG	∼12 kb	∼3.7 kb	High	High	Advantage:	Complete retrograde transport, efficient infection of axon terminals
	Trans-synaptic	Trans-monosynaptic	EnvA+RVdG	∼12 kb	∼3.7 kb	High	High	Advantage:	Complete retrograde transport, efficient infection of axon terminals
			PRV-dTK	∼142 kb	∼50 kb	High	Low	Disadvantage:	Potential retrograde transport
		Trans-multisynaptic	RV	∼12 kb	∼3.7kb	High	High	Advantage:	Complete retrograde transport, efficient infection of axon terminals
			PRV	∼142 kb	∼50 kb	High	Low	Advantage:	Bartha strain shows complete retrograde transport

AAV, Adeno-associated virus; HSV1-H129-dTK, Herpes simplex virus type 1; H129, TK-deleted; VSV, Vesicular stomatitis virus; CAV-2, Canine adenovirus 2; RV, Rabies virus; RVdG, Rabies virus, glycoprotein G-deleted; PRV, Pseudorabies virus.