The first clinical gene therapy, administered in 1990 to a four-year-old girl, aimed to cure a disorder caused by the deletion of the gene for an enzyme crucial to the functioning of the immune system. Identify the enzyme.

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The first clinical gene therapy was given in 1990 to a 4-year-old girl with adenosine deaminase (ADA) deficiency. This disorder is caused due to the deletion of the gene for adenosine deaminase. The enzyme adenosine deaminase is crucial for the immune system to function. Answer: Adenosine deaminase