What is gene therapy? Name the first clinical case where it was used.

Explanation

**Step-by-Step Solution:**

1. **Definition of Gene Therapy**: Gene therapy is a medical treatment that involves the replacement or alteration of faulty genes with healthy ones to treat or prevent diseases. This approach aims to correct genetic disorders by introducing, removing, or altering genetic material within a person's cells.

2. **Mechanism of Gene Therapy**: In gene therapy, a defective or malfunctioning gene (the faulty gene) is replaced with a normal gene (the healthy gene). For example, if a gene that normally produces a specific protein is mutated, gene therapy can insert the correct version of that gene, allowing the body to produce the necessary protein and restore normal function.

3. **First Clinical Case of Gene Therapy**: The first clinical case of gene therapy occurred in 1990. It involved a four-year-old girl named Ashanti DeSilva, who suffered from a genetic disorder known as adenosine deaminase deficiency (ADA deficiency). This condition severely compromises the immune system, leaving individuals vulnerable to infections.

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