The T1/ST2 gene encodes two glycoproteins of the interleukin 1 receptor (IL-1R) family. The larger protein is a membrane-anchored orphan receptor, which is very similar to the type I IL-1R, and whose predominant sites of expression are mast cells and TH2 cells. The smaller, secreted protein corresponds to the extracellular domain of the T1/ST2 receptor and is produced by fibroblasts and osteoblasts in low-quantities and in much higher amounts after stimulation with growth factors, proinflammatory cytokines, and in response to oncogene expression. A putative T1/ST2 ligand has been cloned, but was not able to trigger receptor activation. Two proteins of 18kDa and 32kDa have been identified that could bind the extracellular domain of T1/ST2 and induce NFkB activation. The sequence of these proteins has not as yet been obtained. Studies of Tl-deficient mice demonstrated that Tl is not crucial for the development and function of TH2 and mast cells, but suggested that it might be involved in the TH2-specific cytokine production in response to certain parasitic infections.