Hence, immunohistochemistry was performed with thyroid transcription factor 1 (TTF1) to rule out the possibility of tumor origin in the lung, with paired box gene 8 (PAX8) to rule out origin in the pancreas, with caudal type homeobox 2 (CDX2) to rule out origin in the gastrointestinal tract, and with CD20 to rule out Merkel cell carcinoma. The gene discussed is PAX8; the disease is neoplasm.