NKX6-1 is a homeobox transcription factor participating in the development and regulation of endocrine function of pancreatic islets. NKX6-1 was expressed in 36 of the 44 (82%) primary pancreatic WDNETs, 12 of the 18 (67%) primary duodenal WDNETs, and rarely in pulmonary, gastric, and appendiceal WDNETs. The specificity of using NKX6-1 as a marker for pancreatic and duodenal WDNETs is 93%. Of the 26 metastatic WDNETs, NKX6-1 was expressed only in the tumors of pancreatic origin (sensitivity: 63%, specificity: 100%). The inclusion of NKX6-1 in an immunohistochemical panel will be beneficial for identifying the primary sites of WDNETs.