The demonstration that IDO expression plays a role in the remission of acute MS in the EAE animal model of MS suggested the possibility that this enzyme could be involved in regulating the clinical course of disease [12]; we thus decided to evaluated the expression profile of IDO and IFN-γ and the enzymatic activity of IDO (Kyn/Trp ratio) in correlation with clinical features in RR-MS patients. This evidence concerns the gene IFNG and myeloid sarcoma.