Protein ST2, also known as IL1RL1, is a member of the interleukin 1 receptor family. The gene of ST2 encodes three splice variants: a soluble secreted form (sST2), a transmembrane receptor form (ST2L), and a variant form of ST2 (ST2V). Interleukin-33 (IL-33) has been identified as a functional ligand of ST2L. IL-33 exerts its cellular functions by binding a receptor complex composed of ST2L and IL-1R accessory protein (IL-1RAcP). The interaction of IL-33 and ST2L activates mitogen-activated protein kinases and several biochemical pathways. The end of these reactions is the activation of the inhibitor of nuclear factor-kB (NF-kB) kinase complex, triggering NF-kB activity. sST2 seems to act as a decoy-receptor for IL-33: it binds IL-33 thus subtracting such a molecule from the interaction with ST2L. sST2 is formed by many cells and sST2 level is increased as response to myocardial stress, as well as in inflammatory conditions, including allergic asthma.