Adenosine deaminase deficiency is caused by _________ of the gene.

The correct answer is Option (3) → Deletion

Option (3) Deletion — Correct. ADA deficiency is caused by the deletion of the gene for the enzyme adenosine deaminase. Without this enzyme the immune system does not work properly.

NCERT Extract:
Gene Therapy – “… The first clinical gene therapy was given in 1990 to a 4-year old girl with adenosine deaminase (ADA) deficiency. This enzyme is crucial for the immune system to function. The disorder is caused due to the deletion of the gene for adenosine deaminase. In some children ADA deficiency can be cured by bone marrow transplantation; in others it can be treated by enzyme replacement therapy, in which functional ADA is given to the patient by injection.”