NOS1 and neoplasm: In human, NO is mainly synthesized in the tumour tissue via three nitric oxide synthase (NOS) enzymes: endothelial NOS (eNOS), inducible NOS (iNOS) and neuronal NOS (nNOS), all of which can play a role in tumorigenesis including tumour proliferation, angiogenesis and tumour metastasis (Figure 1).