How retroviruses are being used in genetic engineering.

Explanation

**Step-by-Step Solution:**

1. **Understanding Retroviruses**: Retroviruses are a type of virus that can convert their RNA genome into DNA once they enter a host cell. This unique ability is crucial for their use in genetic engineering.

2. **Retroviral Vectors**: In genetic engineering, retroviruses are commonly used as vectors. A vector is a vehicle that can carry genetic material into a host cell. Retroviral vectors are particularly effective for gene therapy applications.

3. **Components of Retroviruses**: Retroviruses have three main genomic components:
- **GAG**: Encodes for viral proteins.
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