ST2 (also known as T1) is a member of the IL-1 receptor family (IL-1R) and comes in at least three isoforms (membrane bound, soluble and a variant ST2). Its been over a decade since ST2 was reported to be selectively and stably expressed on the surface of Th2 cells and that ST2-specific blocking antibodies or soluble ST2 impairs Th2 cell responses, but until 2005, the ST2 ligand was unknown.