Promoter	Target Cell Type	Size (bp)	Origin (Species)	Key Characteristics
hSyn (Human Synapsin I)	Pan-neuronal	~470 bp	Human (Homo sapiens)	The “gold standard” for AAV; compact and highly specific to mature neurons.
CaMKIIα	Excitatory (Pyramidal)	~1.3 kb	Mouse (Mus musculus)	Drives expression in glutamatergic neurons of the cortex and hippocampus.
TH (Tyrosine Hydroxylase)	Dopaminergic	~2.5 – 9 kb	Human / Rat	Often used in Parkinson’s research. Larger versions provide better specificity but are hard to fit in AAV.
DAT (Slc6a3)	Dopaminergic	~2.0 – 3.5 kb	Mouse / Human	Specific to Dopamine Transporter-expressing neurons; typically used in the Midbrain (SNc/VTA).
GAD65 / GAD67	Inhibitory (GABAergic)	~2.0 – 3.0 kb	Mouse / Rat	Targets the enzymes that synthesize GABA; often require enhancers to reach high expression levels.
ChAT	Cholinergic	~6.4 kb	Mouse (Mus musculus)	Highly specific but very large. Minimal versions (~3.4 kb) exist but may show “leakiness.”
NSE (ENO2)	Pan-neuronal	~1.8 – 2.2 kb	Human / Rat	Traditionally used as a general neuronal marker; stronger than hSyn but sometimes active in non-neuronal cells.

Key Technical Notes:

• The hSyn Advantage: The human Synapsin 1 promoter is favored because its 469 bp length leaves maximum room for large therapeutic genes or complex regulatory elements.
• Minimal vs. Full-length: For many specific promoters like ChAT or GAD, researchers often use “minimal” versions (removing introns or distal enhancers) to fit into AAV. However, this can sometimes lead to ectopic expression (leaking into other cell types).
• Species Crossing: While promoters are derived from specific species (like Mouse CaMKIIα), they generally function effectively across most mammalian models (human, non-human primate, rat) because the transcription factor binding sites are highly conserved.