Furthermore, tumor cells secrete immunomodulatory molecules that inhibit the activity of NK cells such as TGF-β, prostaglandin E2, adenosine or indoleamine 2,3-dioxigenase (IDO) [102], but it is yet unclear whether these molecules also play a role in tumor immune surveillance in the vasculature/circulatory system. This evidence concerns the gene IDO1 and neoplasm.