PEAC is described as a primary lung adenocarcinoma that contains more than 50% enteric differentiation with typical morphology and immunochemistry staining for at least one marker of colorectal cancer (CRC), such as cytokeratin-20 (CK-20), caudal-related homeodomain protein 2 (CDX-2) or mucin 2 (MUC2). The gene discussed is KRT20; the disease is colorectal cancer.