The action of exogenous NO on immune function is complex; for example there is some in vitro evidence for a non-specific immunosuppressive action for NTG [62], and in a rat model of atherosclerosis repeated exposure to NTG acted on macrophages to up-regulate matrix metalloproteinase-9 (MMP-9) and down-regulate tissue inhibitor of metalloproteinase-1 (TIMP-1) [63]. The gene discussed is MMP9; the disease is atherosclerosis.