Target Cell Type	Promoter Name	Size (bp)	Origin (Species)	Key Characteristics
Astrocytes	GFAP	~680 bp – 2.2 kb	Human	The most common astrocyte promoter. The 2.2 kb version (gfa2) is highly specific; the 681 bp version (gfaABC1D) is optimized for AAV.
Astrocytes	ALDH1L1	~1.3 kb – 2.5 kb	Mouse / Rat	Often considered a more “pan-astrocytic” marker than GFAP, as it labels cells in both gray and white matter.
Oligodendrocytes	MBP	~1.3 kb – 1.9 kb	Human / Mouse	Myelin Basic Protein. Drives expression during active myelination; highly specific to mature oligodendrocytes.
Oligodendrocytes	PLP1	~2.4 kb	Mouse / Human	Proteolipid Protein 1. Strong promoter for targeting the myelinating cells of the CNS.
Oligodendrocytes	CNP	~400 bp – 1.0 kb	Mouse / Rat	2′,3′-Cyclic-nucleotide 3′-phosphodiesterase. Active in both pre-myelinating and mature oligodendrocytes.
Microglia	Iba1 (AIF1)	~1.0 kb – 1.7 kb	Human / Mouse	Ionized calcium-binding adapter molecule 1. Note: Iba1 often shows limited specificity in AAV vectors without additional mi-RNA targeting.

Critical Insights for Glial Targeting

• AAV Serotype Choice: Promoting specific expression in glia often requires a combination of the right promoter and the right capsid. For example, AAV5, AAV8, and AAV-PHP.B are frequently paired with the GFAP promoter to target astrocytes efficiently.
• The Microglia Challenge: Microglia are notoriously difficult to target with standard AAVs due to their myeloid origin. Even with an Iba1 promoter, researchers often see low expression levels or unintended expression in other immune cells.
• Compact Astrocyte Targeting: If you are tight on space, the gfaABC1D promoter is a truncated version of the 2.2 kb human GFAP promoter. At only 681 bp, it is nearly as specific as the full-length version but fits much better in an AAV-based system.