<p> A number of receptors for lymphokines, hematopoietic growth factors andgrowth hormone-related molecules have been found to share a common bindingdomain. These receptors are designated as hematopoietin receptors [<cite idref="PUB00006640"/>] andthe corresponding ligands as hematopoietins. Further, hematopoietins have beensubdivided into two major structural groups: Large/long and small/shorthematopoietins.</p> <p> One subset of individual receptor chains that are part of receptor complexes for small hematopoietins are structurally related such that their extracellular parts strictly contain the 200 amino-acids hematopoietin domain (duplicated in IL-3/-5/GM-CSF beta chain receptors KH97/AIC2B and AIC2A). They define a structural subgroup containing the following chains: Interleukin-2 receptor beta chain (IL2RB), Interleukin-2 receptor common gamma chain (IL2RG), Interleukin-3 receptor beta chain (AIC2A), Interleukin-3/-5/GM-CSF receptor common beta chain (KH97/AIC2B), Interleukin-4 receptor alpha chain (IL4RA), Interleukin-7 receptor alpha chain (IL7RA), Interleukin-9 receptor alpha chain (IL9RA). </p> <p> A schematic representation of the structure of these receptors is shown below: </p> <pre> +----------------------------------------xxxxxxx---------------------------+ | C C C C Extracellular XXXXXXX Cytoplasmic | +-|-|-------|--|-------------------------xxxxxxx---------------------------+ | | | | Transmembrane +-+ +--+ </pre> <p> IL4RA, IL7RA and IL9RA are specific alpha chain receptors for IL-4, IL-7and IL-9 respectively, whereas IL2RB is common to IL-2 and IL-15 (IL2RA andIL15RA are not members of the hematopoietin receptor superfamily). IL2RG ispart of IL-2, IL-15, IL-7, IL-9 and IL-4 form I receptor complexes [<cite idref="PUB00006651"/>].KH97/AIC2B chain is part of GM-CSF, IL-3 and IL-5 receptor complexes and inthe mouse, AIC2B can be substituted by AIC2A, an IL-3 specific beta chainreceptor [<cite idref="PUB00006637"/>]. Together with either IL13RA1 or IL13RA2, IL4RA is also partof the IL-13 receptor complex for which IL-4 can compete with IL-13 (IL-4receptor complex form II) [<cite idref="PUB00006657"/>]. </p> Short hematopoietin receptor, family 1, conserved site