HIF1A and neoplasm: The net effect of NO on tumor growth or apoptosis depends not only on its source and relative levels but also on the degree of activation of HIF1-α, heme oxygenase-1 (HO-1) and VEGF in the tumor microenvironment (Tamir and Tannenbaum, 1996; Ambs et al., 1997; Fukumura and Jain, 1998; Wink et al., 1998; Xie and Fidler, 1998).