Beneficial effects of HO-1 modulation have also been described in xenotransplantation models, in which HO-1 gene expression appears functionally associated with xenograft survival [135] . In a mouse-to-rat heart trans-plant model, the effects of HO-1 upregulation could be mimicked by CO administration, suggesting that HOderived CO suppressed the graft rejection [136] . The authors proposed that CO suppressed graft rejection by inhibition of platelet aggregation, a process that facilitates vascular thrombosis and myocardial infarction.