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

# Gene therapy

Gene therapy treats a disease by adding, silencing or correcting a gene inside the patient's cells.

Updated 2026-11-01

## Key facts

- Strategies: add a healthy copy, silence a harmful gene (RNAi, antisense), correct or edit the gene (CRISPR).
- In vivo: the vector is given to the patient (e.g. AAV to liver, muscle or retina). Ex vivo: stem cells or lymphocytes are modified outside the body and reinfused.
- Vectors: adeno-associated viruses (AAV, do not integrate), lentiviruses (integrate, for dividing cells), lipid nanoparticles.
- Only somatic cells are treated; germline editing is not allowed in clinical practice.
- Approved examples: ADA-SCID, spinal muscular atrophy, RPE65 retinal dystrophy, haemophilia, β-thalassemia and sickle cell disease, CAR-T cells.

## In detail

- CAR-T cells are the patient's T cells given, by a lentivirus, a chimeric receptor against a tumour antigen (e.g. CD19 in B-cell leukaemias and lymphomas).
- Risks: immune response to the vector, insertion at a site that activates an oncogene (insertional mutagenesis, seen in early SCID-X1 trials), off-target effects.
- AAVs stay as episomes: the effect lasts in non-dividing tissues (neurons, muscle, retina) but fades in dividing ones.
- Antisense oligonucleotides and small interfering RNAs (siRNA) are 'RNA' gene therapies: they do not change DNA, they alter splicing or degrade an mRNA.
- For sickle cell disease and β-thalassemia there is a CRISPR therapy that reactivates fetal haemoglobin by editing the BCL11A enhancer.

## Clinical notes

- These treatments are very expensive and offered in few centres; access in Latin America is still limited.
- ADA-SCID was the first disease treated with gene therapy (1990).

## Related conditions

- [Adenosine deaminase deficiency (ADA-SCID)](https://durgical.com/en/conditions/adenosine-deaminase-deficiency/)
- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)
- [Duchenne muscular dystrophy](https://durgical.com/en/conditions/duchenne-muscular-dystrophy/)

## Related

- [CRISPR-Cas9 and genome editing](https://durgical.com/en/biochemistry/topics/crispr-cas9-and-genome-editing/)
- [Recombinant DNA and cloning](https://durgical.com/en/biochemistry/topics/recombinant-dna-and-cloning/)
- [RNA processing (cap, poly-A tail and splicing)](https://durgical.com/en/biochemistry/topics/rna-processing/)

## Test yourself

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

## How does in vivo gene therapy differ from ex vivo?

In vivo the vector is given to the patient; ex vivo the patient's cells are modified outside the body and reinfused.

## What special risk do integrating vectors carry?

Insertional mutagenesis: they can insert next to an oncogene and switch it on.

## How does a CAR-T cell work?

It is a patient's T cell modified with a chimeric receptor that recognises a tumour antigen and kills the tumour cell.

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

Sources: [What is gene therapy?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/therapy/genetherapy/) (MedlinePlus, US National Library of Medicine, 2026); [What are genome editing and CRISPR-Cas9?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/genomicresearch/genomeediting/) (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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