69-year-old man comes for evaluation of a 1-week history of difficulty swallowing. He has a 10-year history of acid reflux disease controlled with antacids as needed. He does not smoke cigarettes or drink alcoholic beverages. Physical examination shows no abnormalities. Results of a barium swallow test are positive. Endoscopy shows a fungating mass in the lower portion of the esophagus. A biopsy specimen of the mass is obtained, and an immunohistochemical stain is applied.
The antibodies used in this stain are specific for which of the following proteins?