Gene editing includes a set of technologies used to make targeted changes in DNA, from knocking out a gene to correcting a point mutation or inserting an entirely new sequence. The field has expanded from early programmable nucleases such as zinc finger nucleases and TALENs to CRISPR systems, base editing, prime editing, and now click editing, each offering a different balance of precision, efficiency, and delivery complexity.
Key takeaways
- Gene editing methods differ mainly in how they find a target site and how they effect the desired DNA changes.
- Early platforms such as ZFNs and TALENs rely on engineered proteins, while CRISPR systems use guide RNAs, which makes retargeting simpler.
- Standard CRISPR editing often depends on double-strand breaks and cellular repair pathways such as NHEJ or HDR.
- Base, prime, and click editing were developed to enable more precise changes while reducing dependence on double-strand breaks.
- The best editing method depends not just on the editor itself, but also on delivery route, cell type, and donor template design.
What is the purpose of gene editing?
At a practical level, gene editing usually aims to do one of four things: 1) disable a gene, 2) correct a mutation, 3) insert a defined sequence, or 4) regulate gene activity. In research, that may mean creating a knock-out cell line or adding a fluorescent tag. In developing therapeutics, it may mean repairing a pathogenic variant or engineering immune cells to respond in a targeted way in response to a treatment.
Two repair concepts shape most editing strategies. Non-homologous end joining, or NHEJ, repairs DNA breaks quickly but often introduces small insertions or deletions, making it useful for gene knock-outs. Homology-directed repair, or HDR, uses a donor DNA template with homology arms to write a precise sequence change, making it the classic route for knock-ins and targeted correction.
First-generation programmable nucleases
Before CRISPR became widely accepted as a gene editing tool dominant, the main gene editing toolswere zinc finger nucleases, or ZFNs, and transcription activator-like effector nucleases, or TALENs. Both systems work by combining a programmable DNA-binding protein with the FokI nuclease, which cuts DNA only when paired correctly at the target site.
ZFNs were the first widely used programmable nucleases and demonstrated that site-specific genome editing was possible in mammalian cells and in early clinical settings. Their main drawback is that each new target requires complex protein engineering, which makes design and optimization slower and more specialized than RNA-guided methods. TALENs are easier to reprogram than ZFNs and often show strong specificity, but they are large constructs, which makes delivery harder and can complicate manufacturing workflows.
CRISPR-Cas systems
CRISPR-Cas systems changed the field by separating targeting from nuclease design. Instead of engineering a new protein for each locus, researchers can often retarget the system simply by changing the guide RNA sequence while keeping the same Cas protein.
In the most common format, Cas9 creates a double-strand break at a guide-directed site, after which the cell repairs the break through NHEJ or HDR. This made CRISPR faster to design, easier to scale, and especially useful for multiplex editing and pooled screens. Its limits are now well known: off-target cleavage can occur, some loci are constrained by PAM requirements, and double-strand breaks can trigger unwanted indels, chromosomal rearrangements, or toxicity in sensitive cells.
Base, prime, and click editing
Base editing was developed to make single-nucleotide changes without creating a full double-strand break. These systems fuse a Cas nickase or catalytically impaired Cas protein to a deaminase, allowing direct conversion of one base to another within an editing window. This is powerful for point mutations, but it is not perfectly precise: edits can occur at more than one nucleotide within the editing window, and deaminase-related off-target changes remain an important limitation.
Prime editing expands the range of possible edits by combining a Cas nickase with a reverse transcriptase and a prime editing guide RNA, or pegRNA. It can, in principle, install substitutions, short insertions, and short deletions without requiring a donor DNA template or a double-strand break, although efficiency and optimization remain major practical challenges.
Click editing is a newer platform designed for more controlled genome writing. It combines an RNA-programmable nickase with a DNA-dependent polymerase and an HUH endonuclease domain that covalently tethers a single-stranded “click DNA” template encoding the intended edit. Early reports show that click editing can install substitutions, insertions, and deletions with minimal indels and meaningful editing efficiency in human cells. In simple terms, base editing is efficient but limited in edit type and can be imprecise within its window, prime editing is flexible but often technically demanding, and click editing is emerging as a template-driven alternative for precise small edits without double-strand breaks.
Knock-out, knock-in, and donor templates
Knock-out editing usually relies on nuclease-induced breaks repaired by NHEJ, which disrupt a coding region through small insertions or deletions. This remains one of the simplest and most efficient applications of CRISPR, , TALENs, and ZFNs, especially in discovery research and functional genomics.
Knock-in editing is more demanding because it requires a templated repair process. For small edits, the donor may be a short single-stranded oligo; for larger insertions, researchers may need longer single- or double-stranded templates with carefully designed homology arms. Donor quality matters because poor template design can reduce efficiency, increase byproducts, or limit the size and complexity of the insertion, especially in therapeutic workflows where precise integration is the goal.
Delivery and experimental context
The same editor can behave very differently depending on how it is delivered. Ex vivo editing, where cells are modified outside the body and then returned to the patient, offers tight process control and is widely used in engineered T cells and stem cell programs. Because cells can be screened before infusion, ex vivo workflows are well suited to more complex editing strategies, including multiplex editing and precise knock-ins.
In vivo editing is harder because the editor must be delivered directly to tissues using viral vectors, lipid nanoparticles, or related systems. Payload size, tissue targeting, immune responses, and duration of editor expression all influence which platform is practical; these constraints help explain why compact editors and non-DSB approaches continue to attract attention.
Choosing the right method
There is no single best gene editing technology. The right choice depends on the desired edit, whether the application is research or therapeutic, how the editor will be delivered, and how much off-target formation can be tolerated. A knock-out screen in cultured cells may call for standard CRISPR-Cas9, while precise correction of a pathogenic variant may be better suited to base, prime, or click editing depending on the mutation and delivery constraints.
The overall trend is clear: gene editing is moving from “cut and repair” toward more controlled genome writing. ZFNs and TALENs established the principle, CRISPR made editing scalable, and newer methods such as base, prime, and click editing are steadily expanding what can be changed, how precisely it can be changed, and in which cell types those changes can be made.
