We showed that low mRNA expression of O-linked glycosyltransferases (C1GALT1, B3GNT6, and ST6GALNAC1) was detected in CCA tissues, suggesting that the high expression of the Tn antigen in CCA may be due to the lack of activity of these three glycosyltransferases vis-à-vis promoting Tn antigen modification. Here, ST6GALNAC1 is linked to cholangiocarcinoma.