SMAD4 and pancreatic ductal adenocarcinoma: Gene alteration in the oncogene KRAS as well as tumor suppressor genes TP53/p53, SMAD4/CPC4, and p16/CDKN21 are the most common and major driver genes in pancreatic ductal adenocarcinoma (PDAC) [23,24,25], which contribute to poor prognosis [19,20,21].