What is gene therapy? Illustrate using the example of adenosine deaminase $(ADA)$ deficiency.

(N/A) Gene therapy is a technique for correcting a defective gene through gene manipulation. It involves the delivery of a normal gene into the individual to replace the defective gene,for example,the introduction of the gene for adenosine deaminase $(ADA)$ in an $ADA$-deficient individual.
The adenosine deaminase enzyme is crucial for the normal functioning of the immune system. Individuals suffering from this disorder can be treated by the transplantation of bone marrow cells.
The process involves the following steps:
$1$. Extraction of lymphocytes from the patient's bone marrow.
$2$. Introduction of a functional $ADA$ gene into these lymphocytes using a retroviral vector.
$3$. Re-introduction of these genetically modified lymphocytes into the patient's bone marrow.
Once inside,the gene gets activated,producing functional $T$-lymphocytes and restoring the patient's immune system.