CEACAM5 and digestive system cancer: The utility of widely used serum biomarkers, such as carcinoembryonic antigen (CEA) (19), carbohydrate antigen 19–9 (CA19-9) (20), and anti-p53 antibody (p53-Ab; 21–23), for the early diagnosis and prognosis prediction of gastrointestinal tract cancers is limited (24, 25).