ADA deficiency is an inherited condition that occurs in fewer than one in 100,000 live births worldwide. Individuals with ADA deficiency inherit defective ADA genes and are unable to produce the enzyme adenosine deaminase in their cells. This enzyme is needed to break down metabolic byproducts that become toxic to T cell lymphocytes. Most of the body's cells have other means of removing the metabolic byproducts that ADA helps break down and remain unaffected by ADA deficiency. However, T cell lymphocytes, white blood cells that help fight infection, are not able to remove the byproducts in the absence of ADA.

Without ADA, the toxins derived from the metabolic byproducts kill the T cells shortly after they are produced in the bone marrow. Instead of having a normal life span of a few months, T cells of individuals with ADA deficiency live only a...