SMAD4, also called SMAD family member 4, or DPC4 (Deleted in Pancreatic Cancer-4) is a highly conserved protein present in all human tissues. Signaling from the ligand-activated membrane receptor serine/threonine kinases to nuclear targets is mediated by DPC4. Upon ligand binding, the receptors of the TGF-β family phosphorylate SMAD proteins (SMAD1 and SMAD2). These proteins then move into the nucleus, where they activate transcription. To carry out this function, the receptor activated SMAD1 and 2 require association with DPC4, also known as SMAD4. SMAD4/DPC4 is also implicated as a tumor suppressor, since it is inactivated in more than half of pancreatic carcinomas and to a lesser extent in a variety of other cancers. The lack of SMAD4 expression is present in approximately 80% of cases of pancreatic adenocarcinoma, but rarely in endometrial (0%), colorectal (0%), ovarian (3%), lung (0%), breast (2%) adenocarcinomas, and malignant melanoma (4%). SMAD4 is an important marker for confirming a diagnosis of pancreatic adenocarcinoma.