What is ADA?
- ADA deficiency is one form of SCID (severe combined immunodeficiency), a type of disorder that affects the immune system.

- a malfunctioning immune system leaves the body open to infection from bacteria and viruses
- The disease is caused by a mutation in a gene on chromosome 20. 
- The gene codes for the enzyme adenosine deaminase (ADA). Without this enzyme, the body is unable to break down a toxic substance called deoxyadenosine. 
-The toxin builds up and destroys infection-fighting immune cells called T and B lymphocytes.

READ: http://www.depressionatoz.com/related-disorders/adenosine-deaminase-deficiency.html

- ADA deficiency is an autosomal recessive disorder. Both parents need to pass the defective gene to their child in order for that child to inherit the deficiency.

-  Chronic diarrhea, ear infections, recurrent pneumonia and profuse oral candidiasis commonly occur in these children. SCID cases occur in about 1 of 100,000 births

- Currently, the only treatment for ADA-deficient SCID calls for injecting the patients twice a week with the necessary enzyme, Kohn said, a life-long process that is very expensive and often doesn't return the immune system to optimal levels. These patients also can undergo bone marrow transplants from matched siblings, but matches can be very rare

- People afflicted with this disease often have to live in a sheltered or enclosed environment, so that they're not exposed to infectious agents. 

What is gene therapy?
- Gene therapy is the use of DNA as a pharmaceutical agent to treat disease.
- DNA can be used to supplement or alter genes within an individual's cells as a therapy to treat disease. 
- The most common form of gene therapy involves using DNA that encodes a functional, therapeutic gene in order to replace a mutated gene. 

How does gene therapy work? ** (types of vectors and delivery methods)
- do they only want for ADA? or generally all the vectors and delivery methods used?

Challenges or risks faced in the implementation of gene therapy to treat disorder
-The body’s immune system naturally attacks anything that invades it, so the body may attack the genes being introduced to it by gene therapy or may attack the carrier (vector) if it is a virus. 
- viruses used to deliver genes might recover their ability to cause disease in the body.
-  impossible to control where the introduced gene ends up, there is a certain risk of it ending up in the wrong place and giving rise to leukemia. This is why gene therapy is only used for serious diseases for which there is no good treatment,
- ethic problems
- Gene therapy is short-lived
 In order to achieve long-term benefits of gene therapy, the genes that are inserted into the patient must remain functional after treatment. However, most types of cells that receive new genes during gene therapy eventually die and are replaced by new ones. Currently, researchers have not discovered an effective way to integrate therapeutic genes into a patient's genome. As a result, patients need to undergo multiple treatments of gene therapy in order for it to be effective.