Heme oxygenases (HMOXs) cleaves the heme ring at the alpha-methene bridge to form bilverdin. This reaction forms the only endogenous source of carbon monoxide. HMOX1 is inducible and is thought to have an antioxidant role as it is activated in virtually all cell types and by many types of "oxidative stress" (Poss & Tonegawa 1997). HMOX1 forms dimers/oligomers in the endoplasmatic reticulum. This oligomerization is crucial for the stabilization and function of HMOX1 in the ER (Hwang et al. 2009). HMOX2 is non-inducible.