Immunohistochemistry staining is an important adjunct in distinguishing the diagnosis of endometrial adenocarcinoma from primary colorectal carcinomas. Primary colon cancers are cytokeratin-7 negative and cytokeratin-20 positive, whereas endometrial cancers are cytokeratin-7 positive and cytokeratin-20 negative. The gene discussed is KRT20; the disease is endometrial cancer.