Heme oxygenase (HO) is a microsomal enzyme that catalyzes the oxidation of heme to the antioxidant molecules, biliverdin and carbon monoxide. HO consists of two homologous isozymes, an inducible HO1 and a constitutively expressed HO2. HO1 is induced by a wide variety of stimuli including conditions of oxidative stress, inflammatory agents, transforming growth factor beta and heat shock. The increase in expression of HO1 is thought to be a cellular defense mechanism against oxidative stress since elevated HO could eventually generate more bilirubin, an anti-oxidant. HO1 expression is greatly increased in neurofibrillary tangles in Alzheimer's disease and other neurodegenerative diseases such as progressive supranuclear palsy and subacute sclerosing panencephalitis. Inhibition of HO1 is associated with tissue pathology in atherosclerosis and other inflammatory conditions associated with intravascular thrombosis such as septic shock, hypoxia, and graft rejection.