In 1990, the first clinical gene therapy was administered to a 4-year-old girl. This therapy was aimed at treating the deficiency of which of the following enzymes?

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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 properly. Its deficiency leads to Severe Combined Immunodeficiency (SCID). The therapy involved extracting lymphocytes from the patient's blood, growing them in culture, and introducing a functional ADA cDNA into these cells using a retroviral vector before returning them to the patient's body. Answer: Adenosine deaminase