**Core Concept**
The limbal stem cells are essential for the maintenance and regeneration of the corneal epithelium. A universal marker for these stem cells is crucial for diagnosing and managing corneal surface disorders. The correct marker should be specific to the limbal epithelial cells, allowing for their identification and isolation.

**Why the Correct Answer is Right**
P63 is a transcription factor that is highly expressed in the limbal epithelial stem cells. It plays a crucial role in regulating the self-renewal and differentiation of these stem cells. The expression of P63 is specific to the limbal epithelial cells, making it a reliable marker for their identification. It is also involved in the regulation of other stem cell markers, such as cytokeratins and ABCG2, which are also expressed in limbal stem cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** CD34 is a marker for hematopoietic stem cells and is not specific to limbal stem cells. It is not a reliable marker for the identification of limbal epithelial stem cells.
* **Option B:** ABCG2 is a marker for stem cells, but it is not specific to limbal stem cells. It is expressed in various types of stem cells, including hematopoietic and epithelial stem cells.
* **Option C:** Cytokeratins are expressed in epithelial cells, but they are not specific to limbal stem cells. They are also expressed in other types of epithelial cells, such as skin and gut epithelial cells.

**Clinical Pearl / High-Yield Fact**
P63 is a useful marker for diagnosing limbal stem cell deficiency, which is a common cause of corneal surface disorders. It can also be used to identify limbal stem cells in tissue culture, allowing for their expansion and transplantation.

**Correct Answer:** B. ABCG2 is a marker for stem cells, but it is not specific to limbal stem cells.