CEACAM5 and malignant colon neoplasm: The difference in pH-dependency can also explain why the carcinoembryonic antigen (CEACAM5) extracted from colon cancer tissue carries α-2,6-linked sialic acid instead of the α-2,3-linked sialic acid found in normal tissues (Yamashita et al., 1987; Kobata et al., 1995).