In 1990, the first clinical gene therapy was given to a 4-year-old girl suffering from a severe combined immunodeficiency. This condition, caused by the deletion of a specific gene, severely compromises the function of the immune system. Identify the enzyme that is deficient in this disorder.

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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 enzyme is crucial for the immune system to function. The disorder is caused due to the deletion of the gene for adenosine deaminase. -1 antitrypsin is a biological product used to treat emphysema. Phenylalanine hydroxylase is the enzyme deficient in phenylketonuria. Tyrosinase is the enzyme deficient in albinism. Answer: Adenosine deaminase