Hmox1−/− mice display a number of phenotypes, including an exaggerated response to haemin, splenomegaly that presents with age, a deficit in iron re‐utilization that manifests as an accumulation of iron in the liver and kidney with a corresponding reduction in blood‐borne iron, and almost complete perinatal lethality (Poss & Tonegawa, 1997a, b). This evidence concerns the gene HMOX1 and Splenomegaly.