Relevant topics include: (1) NO and its underlying role in tumor promotion/persistence; (2) basic principles of PDT and how it suppresses solid tumors; (3) iNOS/NO-mediated hyper-resistance to PDT and hyper-aggressiveness of surviving cells; (4) mechanism of iNOS/NO induction by PDT; (5) tumor expansion via PDT-induced bystander effects; and (6) pharmacologic approaches for limiting the negative effects of iNOS/NO after PDT. The gene discussed is NOS2; the disease is neoplasm.