These tumors were called by various names, such as mucin-producing cholangiocarcinoma [1–4], mucin-hypersecreting bile duct tumor [5], and intraductal papillary mucinous tumor of the bile duct [6, 7], and were identified as a biliary counterpart of intraductal papillary mucinous neoplasm (IPMN) of the pancreas. Here, MUC5AC is linked to cholangiocarcinoma.