Gene therapy is a technique used to treat genetic disorders by introducing a functional gene into a patient's cells. This gene can replace a faulty gene or provide a new function that can help in curing or managing the disorder. Among the given options, the correct example of gene therapy is the introduction of the gene for adenosine deaminase (ADA) in persons suffering from Severe Combined Immuno-Deficiency (SCID).

Severe Combined Immuno-Deficiency (SCID)
- SCID is a rare genetic disorder characterized by a severe deficiency in the immune system, leaving affected individuals highly susceptible to infections and diseases.
- The most common form of SCID is caused by a mutation in the ADA gene, which leads to a lack of the enzyme adenosine deaminase.
- ADA is essential for the normal development and function of the immune system, particularly T and B cells.

Introduction of ADA gene in SCID patients
- Gene therapy for SCID involves introducing a functional ADA gene into the patient's cells, typically using a viral vector.
- The viral vector is modified to carry the ADA gene and deliver it to the patient's bone marrow cells. Bone marrow contains stem cells that can give rise to all types of blood cells, including immune cells.
- The modified viral vector is injected into the patient's bone marrow, where it infects the cells and transfers the ADA gene.
- The introduced ADA gene then integrates into the cells' DNA and starts producing the ADA enzyme.
- As a result, the patient's immune cells can now produce ADA, which helps in the normal development and function of the immune system.
- This gene therapy approach can potentially provide a long-lasting cure for SCID, as the introduced gene can continue producing ADA throughout the patient's life.

Advantages of ADA gene therapy
- Gene therapy for SCID offers several advantages over other treatment options such as enzyme replacement therapy or bone marrow transplantation.
- It provides a more targeted and specific treatment by correcting the underlying genetic defect.
- Gene therapy can potentially eliminate the need for lifelong enzyme replacement therapy or immunosuppressive drugs.
- It reduces the risk of complications associated with bone marrow transplantation, such as graft-versus-host disease or rejection.

In conclusion, gene therapy holds great promise for the treatment of genetic disorders like SCID. Through the introduction of a functional ADA gene, it aims to restore the normal function of the immune system in SCID patients, providing a potential cure for this severe condition.