On the other hand, PDT-generated tumour ischaemia can elicit a specific stress response known to activate hypoxia-inducible factor-1 (HIF-1) that can be responsible for the induction of VEGF expression (Ferrario et al, 2000), and activation of early response genes including cyclooxygenase-2 and inducible nitric oxide synthase (Hierholzer et al, 2001). This evidence concerns the gene NOS2 and neoplasm.