Table 7.

Currently-available anterograde trans-neuronal viral tracers

Anterograde tracers	Virus	Properties	Limitations
Polysynaptic
H129-G4[16]	HSV-1 (H129)	Brightest anterograde tracer so far Compatible with fMOST Alternative tracer with red fluorescence Can be used in tree shrew and rat	Potential retrograde labeling caused by axonal terminal pickup High toxicity No starter cell-specificity
H129dTK-TT [15]	HSV-1	The first H129-derived anterograde tracer with fluorescence labeling Polysynaptic tracing from Cre+ neuron population	High toxicity Relatively low labeling intensity Cannot trace from naïve neuron population without Cre
H129-EGFP [8]	HSV-1 (H129)	First H129-derived anterograde tracer with EGFP labeling First H129-derived anterograde tracer initiating polysynaptic tracing from naïve neuron populations	High toxicity Relatively low labeling intensity
Monosynaptic
AAV2/1 [35] AAV2/9 [35]	AAV	Easy access Very low toxicity Compatible with functional assays	No starter cell specificity Very low transneuronal transmission efficiency Very low labeling intensity, requiring a reporter system
H129-dTK [16]	HSV-1 (H129)	First anterograde monosynaptic viral tracer Suitable for both starter neuron specific and nonspecific tracing Having an alternative tracer with green fluorescence	Relatively low labeling intensity in post-synaptic neurons Relatively high cytotoxicity in starter neurons