Carcinoembryonic antigen (CEA), which is encoded by CEA gene (also known as cluster of differentiation (CD) 66e or CEACAM5), is a glycoprotein which has been shown to be expressed in embryonic tissues as well as a vast majority of human cancers, including colorectal, gastric, pancreatic, ovarian, breast and lung cancers, but is absent in normal adult tissues49. Here, CEACAM5 is linked to lung cancer.