Table 1 |.

Key properties of viruses used in neuroscience studies

Family genome architecture and BC	Virus (abbreviation)	Genome size and maximum payload	Onset and duration of expression	Integrating or non-integrating	Directionality of spreada	Toxicity	Tropism	Types of experiments	Refs
Adenoviridae (linear dsDNA, BC I)	Canine adenovirus 2 (CAV-2)	Size: 32 kb	Onset: days to week(s)	Non-integrating	Retrograde	Limited	Preferentially infects nerve terminals (over soma)	Projection specificity; long-term studies	24,97
		Payload: ~30 kb	Duration: months
Herpesviridae (linear dsDNA, BC I)	Herpes simplex virus (HSV)	Size: ~152 kb	Onset: short (hours)	Non-integrating	Local: standard	Dependent on strain, replication competence and purity of vector preparation	Multiple strains capable of infecting CNS, many specific for neurons	Projection and cell type specificity; rapid expression; short-term studies; circuit and molecular perturbation; large payloads	8,12,54, 98–102
		Payload: 30+ kbb	Duration: standard, 5–7 days; LT-HSV, weeks to months		Anterograde: HSV-H129
					Retrograde: LT-HSV
	Pseudorabies virus (PRV)	Size: ~142 kb	Onset: short (hours)	Non-integrating	Bidirectional: Becker	Dependent on strain and replication competence	Preferentially infects nerve terminals (over soma)	Projection specificity; polysynaptic tracing; large payloads	13,58, 103,104
		Payload: 30+ kbb	Duration: variable		Retrograde: Bartha
Parvoviridae (sssDNA, BC II)	Adeno-associated virus (AAV)	Size: ~4.7 kb	Onset: 1–2 weeks	Rare integration (largely episomal)	Local: AAV1, AAV2, AAV5, AAV8, AAV9	Minimal	Many serotypes infect CNS; primarily infects soma	Projection and cell type specificity; long-term studies; circuit and molecular perturbation	10,30, 59–61, 105–107
		Payload: ~4.7 kb	Duration: months		Anterograde: AAV1, AAV9
					Retrograde: AAVrgc
Retroviridae (ssRNA-RT, BC VI)	Lentivirus (LV)	Size: ~9.8 kb (based on HIV-1)	Onset: days to weeks	Integrating	Dependent on pseudotype	Limited	VSV-G coat: local transduction; RbV-G coat: terminals and soma	Larger payloads than accommodated by AAV	10,98, 108–111
		Payload: 8 kb	Duration: months
Rhabdoviridae ((−) ssRNA, BC V)	Rabies virus (RbV)	Size: ~12 kb	Onset: short (~2 days)	Non-integrating	Retro	Dependent on strain and replication competence	G coat: neurons and terminals; EnvA coat: TVA+ cells	Projection specificity; monosynaptic/ polysynaptic tracing	11,56, 77,112
		Payload: 3.7+ kb	Duration: months
	Vesicular stomatitis virus (VSV)	Size: ~11 kb	Onset: short (hours)	Non-integrating	Dependent on pseudotype:	High cytotoxicity	Broadly infective, but dependent on pseudotype	Viral tracing; rapid expression	113–115
		Payload: 6 kb	Duration: toxicity-limited		Anterograde: VSV-G?
					Retrograde: RbV-G
Togaviridae ((+) ssRNA, BC IV)	Sindbis virus	Size: ~12 kb	Onset: short (hours)	Non-integrating	Local	Variable	Local infection for neurotropic strain used in mapping	Barcode-based tracing	116–120
		Payload: ~6 kb?	Duration: toxicity-limited

AAVrg, retrograde-tracing variant of AAV; BC, Baltimore classification; dsDNA, double-stranded DNA; EnvA, avian ASLV type A envelope protein; HSV, herpes simplex virus; HSV-H129, anterograde-tracing variant of HSV; LT-HSV, long-term HSV; RbV-G, rabies glycoprotein, protein G; ssRNA, single-stranded RNA; TVA, avian tumour virus receptor A; VSV-G, VSV glycoprotein, protein G.

^aMore information on infection pattern (circuit-specific, monosynaptic or polysynaptic) is shown in FIG. 2.

^bBased on HSV; for amplicons, this number is theoretically 152 kb.

^cAmong others, with lower efficiency.