What is the cause of adenosine deaminase deficiency in humans?

Explanation

### Step-by-Step Solution:

1. **Identify the Condition**: The condition in question is adenosine deaminase deficiency, a genetic disorder affecting the immune system.

2. **Determine the Genetic Cause**: The deficiency is caused by a mutation in a specific gene known as the ADA gene.

3. **Function of the ADA Gene**: The ADA gene provides instructions for producing the enzyme called adenosine deaminase.

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