The functional ADA cDNA can be introduced into the cells of the patient receiving gene therapy by using a vector consituted by :

Options

A. E.coil
B. Reovirus
C. Retrovirus
D. Agrobacterium

Answer

To solve the question regarding the introduction of functional ADA cDNA into the cells of a patient receiving gene therapy, we will analyze the options provided and determine the most suitable vector for this purpose.

### Step-by-Step Solution:

1. **Understanding Gene Therapy**: Gene therapy is a method used to treat or prevent diseases by inserting genes into a patient's cells. In the context of ADA deficiency, gene therapy aims to introduce a functional copy of the ADA gene into the patient's cells.

2. **Identifying the Condition**: ADA deficiency is a genetic disorder that affects the immune system. The first clinical gene therapy was performed on a patient with this condition, highlighting the importance of finding a suitable method for delivering the ADA gene.

3. **Types of Vectors**: Vectors are vehicles used to deliver genetic material into cells. The options given are:
- A. E. Coli (a bacterium, typically used for cloning)
- B. Rheovirus (a virus, but not commonly used for gene therapy)
- C. Retrovirus (a type of virus that integrates into the host genome, commonly used in gene therapy)
- D. Agrobacterium (a bacterium used primarily in plant genetic engineering)

4. **Evaluating the Options**:
- **E. Coli**: Not suitable for gene therapy in humans as it is primarily used for cloning and protein expression.
- **Rheovirus**: While it is a virus, it is not typically used for gene therapy applications.
- **Retrovirus**: This is a well-established vector for gene therapy as it can integrate the therapeutic gene into the host genome, allowing for stable expression.
- **Agrobacterium**: Mainly used for plant cells, not suitable for human gene therapy.

5. **Conclusion**: The most appropriate vector for introducing functional ADA cDNA into the cells of a patient receiving gene therapy is the **Retrovirus**.

### Final Answer:
The correct answer is **C. Retrovirus**.