HMOX1 and breast cancer: In the spontaneous MMTV-PyMT murine model of breast cancer, macrophage expression of heme oxygenase-1 (HO-1), an enzyme responsible for the breakdown of heme to generate the biologically active products biliverdin, ferrous iron (Fe2+) and carbon monoxide (CO) (92), was demonstrated to play a pivotal role in suppressing an anti-tumor immune response generated by 5-FU (10).