The HO-1 isoenzyme is an early stress response gene regulated by many forms of oxidative stress. The HO-2 isoenzyme is predominantly a constitutive enzyme, which may serve to sequester heme as well as degrade it. All HO enzyme activity results in the degradation of heme and the production of antioxidant bile pigments, which would favor an antioxidant role for the enzyme. In fact, in oxidative stress in vitro, HO-1 is protective (91-94) but within a narrow threshold of overexpression (93,94) in some models, since iron released in the HO reaction may obviate any cytoprotective effect (Fig. 3). So far, HO-2 appears to be beneficial in oxygen toxicity in vivo, but the consequences of HO-2 overexpression have not yet been tested. It will be important to better define the role of each HO isoenzyme in oxidative stress so as to determine whether enhancing these complex systems could alleviate some of the cellular changes seen as a result of oxidative injury. Furthermore, prior to considering therapeutic maneuvers to enhance HO, a complete understanding of the physiologic consequences of HO-1 induction and associated reactions, in each particular setting, will be crucial.