The original World Health Organization (WHO) classification [101] considered mucin production to be evidence of biliary differentiation and so a tumor with unequivocal hepatocellular differentiation and either gland formation by biliary type epithelial cells or a positive mucin stain was considered to be a hepatocellular-cholangiocellular carcinoma. The gene discussed is MUC5AC; the disease is cholangiocarcinoma.