Which of the following is commonly used as a vector for introducing a DNA fragment in human lymphocytes ?

Options

A. Retrovirus
B. pBR 322
C. 1 Phage
D. Ti plasmid

Explanation

To determine which vector is commonly used for introducing a DNA fragment in human lymphocytes, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for a vector that is commonly used to introduce a DNA fragment specifically into human lymphocytes.

2. **Identify the Options**: The options provided are:
- Retrovirus
- PBR322
- One phage
- Ti plasmid

3. **Analyze Each Option**:
- **Retrovirus**: Retroviruses are known to integrate their RNA into the host cell's DNA, making them effective for gene delivery into human cells, including lymphocytes.
- **PBR322**: This is a plasmid vector used in bacterial systems, not typically for human cells.
- **One phage**: Bacteriophages are viruses that infect bacteria, not human cells.
- **Ti plasmid**: This is a plasmid from Agrobacterium tumefaciens used primarily in plant genetic engineering.

4. **Select the Most Appropriate Vector**: Given the analysis, retrovirus is the most suitable vector for introducing DNA into human lymphocytes due to its ability to integrate into the host genome.

5. **Conclusion**: The correct answer is **retrovirus**.

### Final Answer:
The vector commonly used for introducing a DNA fragment in human lymphocytes is **retrovirus**.