CMV early enhancer/chicken β-actin/rabbit β-globin hybrid promoter (CAG)

Synthetic hybrid promoter combining the CMV immediate-early enhancer, the chicken β-actin promoter, and a rabbit β-globin splice acceptor/intron, engineered for very high constitutive expression across mammalian cell types and tissues. One of the strongest promoters available for transgenic animals, viral vectors, and stable cell lines.

Length: 1733 bp

Strength: strong

Tissue: Ubiquitous — active across mammalian cell types, consistently among the strongest constitutive promoters tested though absolute output varies by cell type (Qin et al. 2010).

Expression: constitutive

Origin: Synthetic hybrid — CMV enhancer + chicken β-actin promoter/intron

Characteristics

Fuses the CMV immediate-early enhancer, the chicken β-actin promoter and its 5' splice donor, and a rabbit β-globin splice acceptor/intron into a single ~1.7 kb cassette, combining a strong viral enhancer with a housekeeping promoter's transcriptional machinery (Niwa et al. 1991). In a side-by-side lentiviral reporter comparison across eight mammalian cell lines, the CAG-type hybrid (CAGG) is one of the two consistently strongest promoters tested, on par with EF1A and outperforming CMV, UBC, PGK, and SV40 (Qin et al. 2010).

Applications: Standard choice for generating transgenic mice with ubiquitous, high-level reporter or Cre expression, and for driving transgenes in AAV and lentiviral vectors where maximal constitutive output across diverse cell types is needed, including stable cell-line generation. Used as a benchmark "strong promoter" in promoter-comparison studies.

Limitations: At ~1.7 kb, substantially larger than compact alternatives such as EFS or minimal CMV, competing with the size budget of AAV vectors (~4.7 kb capacity) when combined with a sizeable transgene. Retains the CMV enhancer component, and ubiquitous strong promoters of this type carry a documented risk of transgene silencing over time in some in vivo contexts.

Sequence

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References

Niwa et al. (1991). Efficient selection for high-expression transfectants with a novel eukaryotic vector. Gene - Niwa 1991 CAG
Qin et al. (2010). Systematic Comparison of Constitutive Promoters and the Doxycycline-Inducible Promoter. PLoS ONE - Qin 2010 Promoter Comparison