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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# CRISPR-Cas9 and genome editing

CRISPR-Cas9 is RNA-guided 'molecular scissors' that cut DNA at an exact spot to switch off, correct or insert a gene.

Updated 2026-11-01

## Key facts

- Origin: the immune system bacteria use against viruses (bacteriophages).
- The guide RNA (about 20 nt) leads the Cas9 nuclease to the complementary sequence, which must sit next to a PAM motif (NGG).
- Cas9 makes a double-strand break; the cell repairs it by NHEJ (small insertions or deletions that knock out the gene) or by homologous recombination with a template (precise correction).
- Variants without a double-strand break: base editors and prime editors.
- There is already an approved therapy for sickle cell disease and β-thalassemia that reactivates fetal haemoglobin.

## In detail

- The single guide RNA (sgRNA) joins the search sequence and the scaffold that binds Cas9.
- NHEJ: useful to knock out a gene. Homology-directed repair: to correct a mutation, but it is less efficient and happens mainly in dividing cells.
- Base editors (a nicking Cas9 fused to a deaminase) change C→T or A→G without cutting both strands.
- Dead Cas9 (dCas9) fused to activators or repressors switches genes on or off without changing the sequence.
- Risks: off-target cuts, mosaicism, immune response to Cas9 and the ethics of germline editing.

**Also called**
CRISPR, Gene editing

## Clinical notes

- The approved therapy edits the BCL11A erythroid enhancer ex vivo in the patient's stem cells, which then make fetal haemoglobin.
- In research it is used to build cell and animal disease models and for large-scale gene screens.

## Related conditions

- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)

## Related

- [Gene therapy](https://durgical.com/en/biochemistry/topics/gene-therapy/)
- [DNA repair](https://durgical.com/en/biochemistry/topics/dna-repair/)
- [Recombinant DNA and cloning](https://durgical.com/en/biochemistry/topics/recombinant-dna-and-cloning/)
- [DNA sequencing](https://durgical.com/en/biochemistry/topics/dna-sequencing/)

## Test yourself

Think of the answer, then tap the question to see it.

## What sets the exact spot where Cas9 cuts?

The guide RNA sequence, complementary to the target, which must also sit next to a PAM motif.

## Which repair route is used to knock out a gene after a Cas9 cut?

NHEJ: it introduces small insertions or deletions that usually shift the reading frame.

## Where does the CRISPR-Cas9 system come from?

From the adaptive immune system of bacteria, which stores pieces of viruses to recognise and cut them.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [What are genome editing and CRISPR-Cas9?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/genomicresearch/genomeediting/) (MedlinePlus, US National Library of Medicine, 2026); [CRISPR](https://www.genome.gov/genetics-glossary/CRISPR) (National Human Genome Research Institute (NHGRI), 2026); [What is gene therapy?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/therapy/genetherapy/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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