NOS2 and neoplasm: In the role of NO in cancer tumorigenesis, the low levels of NO generated by NOS lead to tumor initiation and progression by several mechanisms: (i) NO induces DNA damage; (ii) NO interferes with DNA repair; (iii) NO activates oncogenes; (iv) NO decreases apoptosis; (v) NO causes post-translational modification; and (vi) NO causes gene mutations under chronic and malignant conditions, such as the accumulation of mutant p53.