CDX2 and neoplasm: Tumor cells demonstrated positive expression for cytokeratin (CK) 7, thyroid transcription factor-1 (TTF1), and Napsin A. A thorough immunohistochemical (IHC) examination confirmed primary lung origin after tumor cells revealed positive staining for CK7 and TTF-1 and negative staining for CK20, CDX2, and paired box gene 8 (PAX8) IHC stains (Figure 2).