**Core Concept**
Franklin disease, also known as multiple endocrine neoplasia type 1 (MEN1), is a rare genetic disorder characterized by the development of tumors in multiple endocrine glands, including the parathyroid glands, pancreas, and pituitary gland. The neoplastic cells in Franklin disease exhibit uncontrolled cell growth and division due to mutations in the MEN1 gene, which encodes a tumor suppressor protein.

**Why the Correct Answer is Right**
The neoplastic cells in Franklin disease express **CDKN1B**, a cyclin-dependent kinase inhibitor that regulates the cell cycle. The loss of MEN1 protein function leads to the inactivation of CDKN1B, resulting in uncontrolled cell proliferation and tumor formation. The expression of CDKN1B is a key feature of the neoplastic cells in Franklin disease, and its inactivation is a critical step in the development of endocrine tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the neoplastic cells in Franklin disease do not express **CDKN2A**, a different cyclin-dependent kinase inhibitor that is involved in the regulation of the cell cycle.

**Option B:** Incorrect because the neoplastic cells in Franklin disease do not express **HER2**, a receptor tyrosine kinase that is overexpressed in certain types of cancer, such as breast and gastric cancer.

**Option C:** Incorrect because the neoplastic cells in Franklin disease do not express **PD-L1**, a protein that is involved in immune evasion and is often overexpressed in cancer cells.

**Clinical Pearl / High-Yield Fact**
The neoplastic cells in Franklin disease often exhibit a characteristic "parathyroid-pituitary-pancreatic" tumor pattern, with hyperparathyroidism, pituitary adenomas, and pancreatic islet cell tumors being common manifestations of the disease.

**Correct Answer:** C. PD-L1