Mucin 4, a membrane-bound mucin, is the human homolog of the rat sialomucin complex (SMC). The Mucin 4 gene is the predominant mucin gene expressed in the normal urothelium and is also expressed in several normal tissues such as trachea, lung and testis. Dysregulation of mucin 4results in high levels of expression in pancreatic tumors and tumor cell lines. Induction of mucin 4 in pancreatic carcinoma by alltrans-retinoic acid is mediated through the retinoic acid receptor-å signaling pathway. MUC-4 is a very specific (100%) andsensitive (90%) marker of lung adenocarcinomas and is negative for mesotheliomas. MUC4 is expressed in many human adenocarcinomas. It is specifically expressed in lowgrade fibromyxoid sarcoma. Reportedly, MUC-4 expression in invasive ductal carcinoma of the pancreas is an independent factor for poor prognosis.