The hormone which regulates the gene-expression of the target cell is

Options

A. Prolactin
B. Oxytocin
C. Thyroxine
D. Growth hormone.

Answer

To determine which hormone regulates the gene expression of the target cell, let's analyze the options provided: prolactin, oxytocin, thyroxine, and growth hormone.

### Step-by-Step Solution:

1. **Understanding Hormone Functions**:
- **Prolactin**: This hormone is primarily involved in stimulating milk production in mammary glands during pregnancy. It does not play a significant role in gene expression regulation.
- **Oxytocin**: This hormone is secreted by the posterior pituitary and is mainly responsible for uterine contractions during childbirth and milk ejection during breastfeeding. It does not regulate gene expression.
- **Thyroxine**: This hormone is secreted by the thyroid gland. It plays a crucial role in regulating metabolism and energy production. Importantly, thyroxine can influence gene expression by interacting with its receptors inside the cell.
- **Growth Hormone**: Secreted by the anterior pituitary gland, it stimulates growth and metabolism of carbohydrates, fats, and proteins but does not directly regulate gene expression.

2. **Mechanism of Thyroxine**:
- Thyroxine binds to its receptors located inside the target cells.
- Once bound, it forms a hormone-receptor complex.
- This complex then interacts with the promoter region of specific genes, leading to either activation or inhibition of transcription. This is a direct mechanism of regulating gene expression.

3. **Conclusion**:
- Based on the analysis, the hormone that regulates the gene expression of the target cell is **thyroxine**.

### Final Answer:
The hormone which regulates the gene expression of the target cell is **thyroxine**.

---