186 Background: Gene mutations of the succinate dehydrogenase (SDH) complex have recently been involved in the pathogenesis of cancer cells. These mutations often are associated with SDH subunity B loss of activity and overexpression of HIF-1α. The immunoistochemical (IHC) loss of SDHB expression has recently been reported to be a surrogate marker of malignancy in pheocromocytomas and paragangliomas. The aim is the evaluation of SDHB levels expression in advanced midgut carcinoids and investigate its potential role as prognostic marker. Methods: 31 advanced midgut carcinoids were treated in our Istitution, including 24 males and 7 females, aged 55.5 yrs on average (range 19-75). All patients were G1 tumors with stage IV for synchronous liver metastases. 25/31 patients underwent surgical primary resection. IHC evaluation of the SDHB and MIB1 expression were carried out in 20 primary tumors (T) and 19 liver metastases (M). In 11 patients SDHB and MIB1 were tested in both T and M. SDHB was assessed according...