ADA is a autosomal recessive disorder which means that both the parent have the defective copy of the allele to expression of this disorder in the child. Its deficiency is caused by a mutation in a gene on the q arm of chromosome 20. This gene codes an enzyme called Adenosine Deaminase (ADA). Without the proper functioning of this enzyme, the body is unable to break down the substance Deoxyadenosine. So the toxin builds-up in the body and destroys the T and B lymphocytes, which are responsible to fight against different infections in the body. This rare genetic disorder is one form of the Severe Combined Immunodeficiency (SCID).