Heme oxygenase is the rate-limiting enzyme in the heme degradative pathway. There are 2 isozymic forms of heme oxygenase, an inducible heme oxygenase-1 (HMOX1) and a constitutive heme oxygenase-2 (HMOX2). Down-regulation of HMOX2 is associated with the increased expression of HMOX1 in human cell lines. HMOX2 is also named as HO-2 and could be implicated in the production of carbon monoxide in brain where it could act as a neurotransmitter.