## **Core Concept**
Paget's disease of the breast is a rare form of breast cancer that primarily affects the skin of the nipple and, sometimes, the areola. It is often associated with an underlying breast cancer, typically ductal carcinoma in situ (DCIS) or invasive breast cancer. A key diagnostic feature is the presence of Paget cells.

## **Why the Correct Answer is Right**
The correct answer, **HER2/neu**, is a marker that can be positive in Paget's disease of the breast. Paget cells, which are the malignant cells found in Paget's disease, often express **HER2/neu**, a protein that can promote the growth of cancer cells. The expression of HER2/neu in Paget's disease is similar to that seen in some types of breast cancer, highlighting the connection between Paget's disease and underlying breast malignancies.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *p53* is a tumor suppressor gene, and mutations in p53 can lead to various cancers, including breast cancer. However, p53 is not specific to Paget's disease of the breast.
- **Option B:** *GCDFP-15 (Gross Cystic Disease Fluid Protein 15)* is actually a marker associated with apocrine differentiation and can be found in Paget cells, making it relevant to Paget's disease of the breast.
- **Option D:** While estrogen receptor (ER) and progesterone receptor (PR) status are important in breast cancer, they are not specific markers for Paget's disease.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Paget's disease of the breast often presents with eczema-like changes to the nipple and areola. A biopsy showing Paget cells, which are large, pale-staining cells with distinctive nuclei, is diagnostic. **GCDFP-15** is indeed a useful marker for diagnosing Paget's disease, as it is often positive in Paget cells, reflecting their apocrine origin.

## **Correct Answer:** C. HER2/neu