An episome is a DNA element that can replicate independently of a cell’s main chromosomes, persisting inside the nucleus as a separate, self-sustaining genetic unit. The term was introduced by François Jacob and Elie Wollman in 1958 to describe certain bacterial elements, particularly the F (fertility) factor, that could exist either integrated into the chromosome or floating free in the cell.1PubMed. Hybridizing bacteria, crossing methods, cross-checking arguments: the transition from episomes to plasmids (1961-1969) Since then, the concept has expanded well beyond bacteria. Episomes now figure centrally in virology, cancer biology, gene therapy, and stem cell research, and the functions they perform range from enabling drug resistance in parasites to driving tumor evolution in human cancers.
The Original Bacterial Episome
The story starts with bacteria. In Escherichia coli, the F factor is the classic episome. When it floats freely in the cytoplasm, it replicates on its own and can be transferred to other bacteria during conjugation, the bacterial equivalent of mating. But under certain conditions the F factor integrates into the host chromosome, turning the cell into what microbiologists call a high-frequency recombination (Hfr) strain. That integration is not random. It tends to happen at specific chromosomal sites where short matching DNA sequences, called insertion sequences, sit on both the F factor and the chromosome. Recombination between these matching stretches stitches the F factor into the chromosome at a predictable location and orientation.2PubMed Central. Chromosomal integration of F’ factors in recombination-deficient Hfr strains of Escherichia coli
This ability to toggle between autonomous and integrated states was exactly what Jacob and Wollman were trying to capture with the word “episome.” Over the following decade, the term gradually merged with “plasmid,” and today most microbiologists use “plasmid” for any self-replicating circular DNA in bacteria, reserving “episome” mainly for contexts where the element’s dual lifestyle matters. But the underlying biology has not changed. Plasmids and related episomes remain key vehicles for horizontal gene transfer, shuttling genes for antibiotic resistance, metabolic capabilities, and other traits between distantly related bacterial species.3PubMed Central. Eco-Evolutionary Dynamics of Episomes among Ecologically Cohesive Bacterial Populations Research on these elements has been heavily skewed toward antibiotic resistance, but their role in natural microbial communities is far broader. They allow bacteria to rapidly assemble and share novel functions in ways that ordinary chromosomal mutation cannot match.
Viral Episomes and Chronic Infection
In virology, the term “episome” refers to viral DNA that persists in the nucleus of an infected cell without integrating into the host genome. Several medically important viruses maintain themselves this way, and the episomal form is often what makes chronic infection so hard to cure.
Hepatitis B virus (HBV) provides a vivid example. After HBV enters a liver cell, its partially double-stranded DNA genome is converted into a small, circular, fully double-stranded molecule called covalently closed circular DNA, or cccDNA. This cccDNA sits in the nucleus as a plasmid-like episome and serves as the master template for all the virus’s RNA molecules and, ultimately, for new virus particles.4Gut. HBV cccDNA: viral persistence reservoir and key obstacle for a cure of chronic hepatitis B Current antiviral drugs can suppress HBV replication effectively, but they rarely eliminate cccDNA. As long as even a few copies of this episome survive in liver cells, the virus can bounce back once treatment stops. That persistence is the central reason chronic hepatitis B remains so difficult to cure.
Human papillomavirus (HPV) follows a somewhat different strategy. In early-stage cervical lesions, HPV typically maintains its genome as a circular episome in the basal cells of the epithelium. Research on the W12 cell line, which carries HPV-16 episomes, showed that even the episomal form alone can drive phenotypic progression. After extended passage, cells harboring only episomal HPV displayed features resembling high-grade precancerous lesions and eventually acquired the ability to invade surrounding tissue in laboratory models.5Cancer Research. In vitro Progression of Human Papillomavirus 16 Episome-Associated Cervical Neoplasia Displays Fundamental Similarities to Integrant-Associated Carcinogenesis This finding challenged the long-held assumption that only HPV integration into the host chromosome drives cancer. The episomal form, it turns out, can be dangerous on its own.
How Viruses Keep Their Episomes from Getting Lost
A free-floating circular DNA in the nucleus faces a practical problem every time the cell divides: chromosomes get carefully sorted into the two daughter cells by the mitotic spindle, but an episome has no centromere and no automatic ticket to ride. Viruses that maintain long-term episomes have evolved clever solutions.
Kaposi’s sarcoma-associated herpesvirus (KSHV) uses a protein called LANA to solve this partitioning problem. LANA acts as a molecular bridge. One end of the protein binds to a specific repeat region on the viral episome, while the other end grabs onto host chromosomal proteins, including histones and other chromatin-associated factors like Brd4, Bub1, and CENP-F.6PubMed Central. KSHV LANA—The Master Regulator of KSHV Latency By tethering itself to the condensed chromosomes during cell division, the viral episome essentially hitchhikes into both daughter cells. No tether, no persistence.
Epstein-Barr virus (EBV) takes a strikingly similar approach. Its EBNA-1 protein binds the viral origin of replication and simultaneously latches onto host metaphase chromosomes through at least three separate binding domains.7PubMed Central. Mapping EBNA-1 domains involved in binding to metaphase chromosomes EBNA-1 also interacts with the host protein Brd4, though this interaction appears to be linked to gene activation rather than segregation itself.8PubMed Central. The EBNA1 protein of Epstein-Barr virus functionally interacts with Brd4 The convergence of KSHV and EBV on such similar chromosome-tethering strategies suggests this is an effective and perhaps inevitable solution to the episome-persistence problem in dividing cells.
Episomes and the Immune System
Foreign DNA in the cell nucleus is not invisible to the immune system. Cells have sensors that detect DNA that should not be there, and one of the most important is a protein called cGAS, which triggers an innate immune alarm when it encounters unusual DNA in the cytoplasm. Viruses that maintain episomes have to deal with this surveillance.
KSHV has found a direct countermeasure. Certain forms of the LANA protein that localize to the cytoplasm physically bind to cGAS and block the signaling cascade that would normally alert the immune system. By inhibiting this pathway, LANA helps KSHV avoid immune detection during both latent and active infection.9PubMed Central. Cytoplasmic isoforms of Kaposi sarcoma herpesvirus LANA recruit and antagonize the innate immune DNA sensor cGAS
HPV’s relationship with cGAS is more complex and less straightforwardly evasive. In cells stably maintaining HPV episomes, cGAS levels are actually elevated compared to uninfected cells, and the downstream signaling components of the cGAS pathway remain functional.10PLOS Pathogens. Human papillomaviruses sensitize cells to DNA damage induced apoptosis by targeting the innate immune sensor cGAS Rather than silencing the sensor entirely, HPV appears to manipulate the consequences of that signaling, redirecting the pathway to sensitize cells to DNA-damage-induced death. The full picture of how HPV episomes coexist with an active innate immune sensor is still being worked out, but it underscores that each virus has developed its own nuanced relationship with host defenses.
Extrachromosomal DNA in Cancer
Some of the most striking episome biology in recent years has come from cancer research, and it involves DNA elements that are not viral at all. Tumor cells frequently carry small circular pieces of their own DNA that have broken free from chromosomes and replicate independently in the nucleus. These elements, usually called extrachromosomal DNA or ecDNA, function as episomes in every practical sense.
A landmark whole-genome sequencing study across 17 cancer types found ecDNA in nearly half of human tumors but almost never in normal cells.11PubMed Central. Extrachromosomal oncogene amplification drives tumor evolution and genetic heterogeneity The genes most commonly amplified on these circles were oncogenes, the growth-promoting genes that help tumors proliferate. A subsequent analysis of over 3,200 cancer patients confirmed the pattern: ecDNA amplification occurs frequently across most cancer types, and oncogenes are highly enriched on these elements.12PubMed Central. Extrachromosomal DNA is associated with oncogene amplification and poor outcome across multiple cancers
What makes ecDNA particularly dangerous is the way it behaves during cell division. Because these circles lack centromeres, they do not segregate evenly into daughter cells the way chromosomes do. Instead, they distribute randomly, which means some daughter cells end up with many copies and others with few. This unequal distribution generates enormous genetic diversity within a single tumor, giving cancer cells more raw material for natural selection to act on. Mathematical modeling and direct measurements confirm that ecDNA amplification increases both oncogene copy numbers and intratumoral heterogeneity more effectively than the same genes amplified on chromosomes.
There is also a chromatin angle. When oncogenes sit on ecDNA rather than on a chromosome, the DNA is packaged more loosely, with far greater accessibility to the cellular machinery that reads genes. Pan-cancer analyses show that oncogenes encoded on ecDNA rank among the most highly expressed genes in the entire tumor.13PubMed Central. Circular ecDNA promotes accessible chromatin and high oncogene expression So ecDNA amplification delivers a double hit: more copies of the oncogene and higher expression per copy. The clinical consequence is worse patient outcomes, which is why ecDNA is now being seriously investigated as both a biomarker and a therapeutic target.
Episomal Vectors in Gene Therapy
The same properties that make viral episomes persistent in infected cells, namely stable replication without chromosomal integration, have attracted considerable interest in gene therapy. Traditional gene therapy vectors, particularly those based on retroviruses and lentiviruses, work by inserting a therapeutic gene directly into the patient’s chromosomes. This can produce long-lasting gene expression, but it also carries a risk: if the vector lands in or near a critical gene, it can disrupt normal cell function and, in rare cases, trigger cancer.
Episomal vectors sidestep this problem. Because they replicate as separate circles in the nucleus and do not integrate, there is a significantly lower risk of the kind of insertional damage that has derailed some gene therapy trials.14PubMed Central. Advances in the Development and the Applications of Nonviral, Episomal Vectors for Gene Therapy Many of these vectors borrow the replication machinery of EBV, using the oriP sequence and EBNA-1 protein to maintain themselves in human cells. The approach also tends to provoke weaker innate immune reactions and is cheaper to manufacture than viral vector systems.15OBM Genetics. Episomal Vectors: Principle, Utility, and Application
The tradeoff is that episomal vectors are gradually lost from dividing cells over many generations, since they lack the robust partitioning systems of true chromosomes. For conditions where long-term but not necessarily permanent expression is needed, or where the target cells divide slowly, this can be perfectly acceptable. For rapidly dividing tissues, researchers are still working to improve episomal retention rates.
Making Stem Cells Without a Genetic Footprint
One of the most successful real-world applications of episomal technology is in generating induced pluripotent stem cells, or iPSCs. These are adult cells that have been reprogrammed back to an embryonic-like state by introducing a small set of genes, often called Yamanaka factors after Nobel laureate Shinya Yamanaka. The original reprogramming methods used retroviruses, which meant the reprogramming genes permanently integrated into the cell’s genome. That foreign DNA could interfere with later experiments or, in clinical applications, pose safety concerns.
Episomal plasmids offered an elegant workaround. Researchers deliver the reprogramming genes on circular DNA that replicates for a while in the cell, long enough to push the cell into a pluripotent state, and then is gradually lost as the cell divides. The result is a “footprint-free” iPSC: a cell that has been fully reprogrammed but retains no trace of the vectors used to get there.16PubMed. Generation of Footprint-Free Induced Pluripotent Stem Cells from Human Fibroblasts Using Episomal Plasmid Vectors This approach has been refined steadily. By combining episomal delivery with small molecules and optimized culture conditions, one group achieved roughly a 70-fold increase in reprogramming efficiency compared to earlier episomal methods.17PubMed Central. Efficient Feeder-Free Episomal Reprogramming with Small Molecules
The technique has now been applied to a range of cell types, including mesenchymal stem cells from umbilical cord tissue, to produce clinical-grade iPSCs that exhibit normal pluripotency markers and carry no residual episomal DNA.18PubMed. Generation of footprint-free human induced pluripotent stem cell line (SCIKFi001-B) from cGMP grade umbilical cord-derived mesenchymal stem cells (UC-MSCs) using episomal-plasmid based reprogramming approach For disease modeling, drug screening, and eventually cell-based therapies, footprint-free iPSCs are now the standard in many laboratories.
Episomes in Yeast and Industrial Biotechnology
Episomal DNA is not limited to pathogens and medicine. Baker’s yeast, Saccharomyces cerevisiae, naturally harbors a small circular DNA called the 2-micron plasmid, which has become one of the workhorses of industrial biotechnology. Researchers use vectors based on this plasmid to produce recombinant proteins and to optimize synthetic metabolic pathways. Because the plasmid maintains itself at high copy number, genes placed on it get expressed at much higher levels than if they were inserted into a chromosome.
Copy number itself can be tuned. By engineering the selectable marker on the plasmid to be less stable or less strongly expressed, researchers can force cells to maintain more copies of the plasmid in order to survive, effectively boosting the output of whatever protein the plasmid encodes.19FEMS Yeast Research. Enhancing the copy number of episomal plasmids in Saccharomyces cerevisiae for improved protein production This kind of fine-tuning is much harder to achieve with genes integrated into chromosomes, where copy number is fixed. The flexibility of episomal systems makes them especially attractive for synthetic biology projects where pathway optimization requires rapid iteration.
Episomes in Parasites and Eukaryotic Genome Plasticity
Episomes also appear in unexpected corners of eukaryotic biology. The parasite Leishmania, which causes the tropical disease leishmaniasis, amplifies drug-resistance genes on extrachromosomal circular DNA. Early studies showed that these amplified DNA circles could be either stable or unstable in resistant cell lines, but all were clearly extrachromosomal rather than integrated into the main chromosomes.20PubMed. Stable amplified DNA in drug-resistant Leishmania exists as extrachromosomal circles This gives the parasite a fast, reversible way to develop resistance: amplify the right genes on episomes when drug pressure is present, and let them drift away when it is not.
More broadly, extrachromosomal circular DNA elements appear across eukaryotes, from yeast to human cells. Many of these circles contain sequences derived from repetitive regions of chromosomes, likely generated through recombination events. Their abundance and variety hint at a general mechanism for genome plasticity: cells can explore genetic possibilities on disposable, circular test platforms without permanently altering their chromosomes. In some organisms, changes in these extrachromosomal elements are associated with shifts in developmental patterns or phenotype in response to environmental conditions.
Targeting Viral Episomes with Gene Editing
The persistence of viral episomes like HBV cccDNA has motivated researchers to try destroying them directly. CRISPR-Cas9 gene editing, which can be programmed to cut specific DNA sequences, has been aimed squarely at cccDNA in laboratory settings. In one study, guide RNAs targeting conserved regions of HBV efficiently reduced cccDNA levels and viral protein production in cell culture, and when tested in a mouse model carrying HBV cccDNA, injection of the editing components lowered both cccDNA and viral protein levels.21PubMed. Targeting hepatitis B virus cccDNA by CRISPR/Cas9 nuclease efficiently inhibits viral replication
A separate study reported even more dramatic reductions, with cccDNA levels falling by over 90% within about five weeks of treatment in cell culture.22Scientific Reports. CRISPR/Cas9 cleavage of viral DNA efficiently suppresses hepatitis B virus These are still early-stage results, and significant challenges remain before CRISPR-based cccDNA destruction could reach the clinic, including safe delivery of the editing machinery to enough liver cells and avoiding off-target cuts in the human genome. But the approach represents a fundamentally different strategy from current antiviral drugs, which suppress new virus production without touching the episomal reservoir that keeps the infection alive.
Artificial Chromosomes and Episomal Engineering
The ability to maintain large DNA constructs as stable episomes in human cells has also opened up possibilities in chromosome engineering. Yeast artificial chromosomes, or YACs, can carry hundreds of thousands of base pairs of DNA, far more than a typical plasmid. Researchers demonstrated that by adding the EBV origin of replication to a YAC, constructs as large as 660 kilobases could be maintained as stable episomes in human cells expressing EBNA-1.23PubMed Central. Stable episomal maintenance of yeast artificial chromosomes in human cells This is large enough to carry entire human genes along with their full regulatory sequences, which is something smaller vectors cannot do.
Such episomal YACs bridge the gap between simple gene delivery and the kind of large-scale genome engineering needed for complex diseases where multiple genes or distant regulatory regions are involved. They avoid the safety concerns of chromosomal integration while still providing stable, long-term presence of the delivered genetic material. As the tools for constructing and delivering these large episomal constructs improve, they are likely to play a growing role in both research and therapeutic applications where conventional vectors fall short.