Immunohistochemistry (IHC) is central to resolving this dilemma: CRC typically shows CK7 negativity with CK20, CDX2, and SATB2 positivity, whereas primary lung adenocarcinomas are usually TTF-1 and CK7 positive, with Napsin A expression [1, 2]. The gene discussed is TTF1; the disease is colorectal carcinoma.