Gene therapy is inserting, deleting or replacing a gene in the genome of a patient's cell to treat a disease or disorder.

Gene therapy can be used to treat genetic disorders, cancers, cystic fibrosis, diabetes, haemophilia, etc.

There are three types of gene therapy:
(1) Removing or replacing faulty genes responsible for causing a disease
(2) Inactivating mutated genes
(3) Making a diseased cell detectable by the immune system

The technology of gene therapy has two main facets:
(1) A functional copy of a gene required to synthesise the desired product inside the cell
(2) A vector, such as a liposome or inactivated virus, needed to transfer these functional genes to the host

Gene therapy is approved for some diseases, for example, onasemnogene abeparvovec is used to treat children with spinal muscular dystrophy and voretigene neparvovec for retinal dystrophy.