To test the hypothesis that MUC4 and MUC5AC are tumour-associated mucins in BTC, we characterised expression of MUC4, MUC5AC and MUC5B (present in normal biliary epithelium) by immunohistochemistry in archival biliary tissue and by western blotting and real-time reverse transcriptase–polymerase chain reaction (rt RT–PCR) in bile and serum samples. Here, MUC5AC is linked to neoplasm.