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. Here, IL1RL1 is linked to arthritic joint disease.