NOS2 and neoplasm: With these gene and protein expression profiles, now we know that MDSCs utilize a number of mechanisms to suppress both the innate and adaptive responses of anti-tumor immunity, mostly through the direct inhibition of T cell activation and expansion, including a high level of arginase 1 (ARG1), inducible nitric oxidase (iNOS) [18], or reactive oxygen species (ROS) [19] production, as well as indoleamine 2,3-dioxygenase (2,3-IDO) activity [20].