Obviously, an alternative explanation for the difference in arthritis severity observed between ST2 and IL-33 KO mice might lie with the existence of another ligand for ST2, although we are not aware of data designating a candidate molecule for this function and we have been unable to demonstrate any substantial binding interaction between ST2 and any other IL-1 family member besides IL-33. The gene discussed is IL1RL1; the disease is Arthritis.