EF-1 Alpha Promoter (EF1a): What It Is and How It Compares With CMV

Short answer

The EF-1 alpha promoter (also written EF1a, EF1 alpha or EF-1α) is a strong constitutive mammalian promoter derived from the human elongation factor 1 alpha gene. Researchers often choose it over the CMV promoter when a transgene must keep being expressed over weeks, because CMV-driven transgenes often become silenced. “EF1a” is not a single sequence, though: published human EF1a promoter clones differ in length and in activity.

Definition and scope

Addgene’s molecular biology reference lists EF1a as a constitutive, mammalian promoter, described as a “strong promoter from human elongation factor 1 alpha”; CMV is listed alongside it as a constitutive mammalian promoter from human cytomegalovirus. The gene behind the promoter is EEF1A1, which encodes human elongation factor 1 alpha. In plasmid catalogues, papers and vendor pages the same element appears as EF1a, EF1α, EF-1 alpha and EF1 alpha. This page treats all of these as the same family of promoters, which is exactly why the naming needs care (see below).

A promoter is the DNA element that drives transcription of the sequence placed after it. For the wider design context, including backbone, marker and tag choices, see plasmid design and plasmid vectors.

How it is used in practice

EF1a is typically placed upstream of a gene of interest in an expression plasmid or lentiviral transfer plasmid so that the gene is expressed in a wide range of mammalian cells. Two sourced examples show the reasoning researchers give:

• Stable expression in human cell lines. Skalamera et al. (2012) chose EF1a over CMV because it is “less prone to silencing and provides more stable long term expression”, and describe it as providing stable long-term constitutive expression of most transgenes tested in human cell lines, while noting it is “not as strong in some cell lines”.
• Reprogramming. A lentiviral plasmid deposited at Addgene (pLVH-EF1a.AcGFP-P2A, plasmid 258553) states that the depositor replaced CMV with EF1a because the CMV promoter is silenced prematurely during reprogramming, and that EF1a is less prone to silencing and supports high-titer lentivirus packaging. This is the depositor’s description, not a controlled trial.

For how lentiviral and other viral vectors deliver a transgene, see viral vector types. To read out promoter activity, a reporter gene such as luciferase or GFP is the usual tool; the strong constitutive CAG promoter is another common choice for the same job.

EF1a vs CMV: what the evidence shows

The most concrete head-to-head data in the sources comes from one study in one cell line, so read it as an illustration of a known problem rather than a universal ranking.

Point of comparison	CMV promoter	EF1a promoter	Source
Class (Addgene reference)	Constitutive, mammalian, from human cytomegalovirus	Constitutive, mammalian, from human elongation factor 1 alpha	Addgene Molecular Biology Reference
Silencing	Transgenes “often become silenced”; the authors call this a well-documented phenomenon and did not investigate the mechanism	Described as less prone to silencing	Skalamera et al. 2012
MCF10A cells, GFP-positive fraction after sorting	CMV constructs (NEK6, CCNE1, PCNA) lost 30-60% of the GFP-positive proportion within 2 days	EF1a constructs stayed above 80% GFP-positive for up to 15 days (NEK6, CCNE1) and 34 days (PCNA)	Skalamera et al. 2012
Strength	Not compared in the cited sources	Strong in most human lines tested, “not as strong in some cell lines”	Skalamera et al. 2012

The MCF10A result applies to those three constructs in that cell line. It does not show the same loss in every cell type, and the study did not test the mechanism of silencing.

Not all EF1a promoters are equal

Zheng and Baum (2014) cloned three human EF1a promoters into the same luciferase vector and compared them. The three differ in length: EF1a-1 is 1197 bp (from pEAK8), EF1a-2 is 1272 bp (from pEF-BOS, and includes an extra 81 bp of SV40 small antigen sequence), and EF1a-3 is 1191 bp (from Invitrogen pEF1/myc/ER/GFP).

• EF1a-3 was about 50-fold lower in A5 cells, 145-fold lower in HSG cells, 80-fold lower in HSY cells and 500-fold lower in 293 cells.
• In rat submandibular gland it was about 250-fold lower than EF1a-1.
• EF1a-3 carried 13 point mutations relative to GenBank J04617, which the authors suggest explains the difference.

The same paper notes that CMV promoter fragments also vary by vector (589, 646, 742, 795, 817 and 1650 bp), without testing their function, and concludes that promoters of the same name may not be equivalent in driving transgene expression.

A checklist for methods sections

This checklist is an editorial synthesis of the two studies above, not a published standard. Because the same name covers different sequences, record:

1. The plasmid or vector name and its repository ID (for example an Addgene number), not just “EF1a”.
2. The promoter length in bp and whether any intron or extra sequence is included.
3. The cell type used, since strength differs between lines.
4. The time point of the assay, since silencing is a time-dependent effect.
5. The reference sequence the promoter was checked against, if you sequenced it.

Common misunderstandings

• “EF1a never silences.” The sources say it is less prone to silencing than CMV, not immune.
• “Any EF1a plasmid will behave the same.” Zheng and Baum found up to about 500-fold differences between human EF1a clones.
• “EF1a is always stronger than CMV.” The cited sources compare stability of expression, and note EF1a is weaker in some cell lines.

FAQ

What is the EF1 alpha promoter?

It is a constitutive mammalian promoter derived from the human elongation factor 1 alpha gene, used to drive transgene expression in many cell types (Addgene Molecular Biology Reference).

Are EF-1 alpha, EF1a and EF1 alpha the same promoter?

They are different spellings of the same promoter family, but individual clones vary in length (1191 to 1272 bp in one comparison) and activity, so specify the exact construct.

Why use EF1a instead of CMV?

Because CMV-driven transgenes often become silenced, and EF1a was reported to give more stable long-term expression in human cell lines in the studies cited here.

Sources

• Skalamera D et al. (2012). PLoS One 7(12):e51733. doi:10.1371/journal.pone.0051733. pmc.ncbi.nlm.nih.gov/articles/PMC3520899. Read 2026-11-08.
• Zheng C, Baum BJ (2014). Int J Med Sci 11(5):404-408. doi:10.7150/ijms.8033. medsci.org/v11p0404.htm. Read 2026-11-08.
• Addgene, plasmid 258553 pLVH-EF1a.AcGFP-P2A. addgene.org/258553. Read 2026-11-08.
• Addgene, Molecular Biology Reference: Promoters. addgene.org/mol-bio-reference/promoters. Read 2026-11-08.