HMOX1 and atherosclerosis: While it appears to be clear that HO-1 functional activity is important for its vascular protection as will be discussed in Section “HO System, Enzymatic Byproducts and Protection Against Atherosclerosis,” there is a possibility that various stimuli or microenvironments could lead to changes in the specific enzymatic activity, which is the HO-1 enzymatic activity/HO-1 protein.