Interleukin-2 receptor alpha chain (also called TAC antigen, P55 and mainly CD25) is a protein involved in assembly of high-affinity Interleukin-2 receptor, consisting of alpha (IL2RA), beta (IL2RB) and the common gamma chain (IL2RG). As the name indicates, this receptor interacts with pleiotropic cytokine called Interleukin-2, which effect is mainly important for immune homeostasis. Genetics The human protein Interleukin-2 receptor subunit alpha is encoded by a gene called IL2RA with a length around 51,6 kb. Alternative names for this protein coding gene are IL2R, IDDM10 and TCGFR. Location of IL2RA in human genome is on the short arm of 10th chromosome (10p15.1). Several frequent point mutations, single nucleotide polymorphism (SNP), have been identified in or in close proximity to IL2RA gene in the population. These SNPs have been linked mainly to susceptibility to immune dysregulation disorders, with majority found in research on multiple sclerosis (MS) and type 1 diabetes mellitus. IL2RA gene orthologues with identical protein functionality are relatively abundant and constant among animal species, especially in mammals subgroups. Moreover, conserved homologs of this gene are in mouse, rat, dog, cow, chimpanzee and Rhesus monkey. Expression CD25 is expressed broadly among leukocytes. It can be found especially upon activation on B cells, NK (natural killer) cells, thymocytes, and some myeloid lineage cells (e.g. macrophages, dendritic cells). Nonetheless, highest surface expression of this protein is on regulatory T cells (Tregs), where CD25 is expressed constitutively, especially on a subset classified as naturally occurring Tregs. Therefore IL2RA has been used as a marker to identify CD4+FoxP3+ regulatory T cells in mice, it has been found that a large proportion of resting memory T cells constitutively express IL2RA in humans. Moreover, high expression of CD25 is also on TCR activated conventional T cells (both CD8+ and CD4+ T lymphocytes), where it is considered as one of T cell activation markers. Additionally, expression of IL-2 receptor subunit alpha can be found in non-lymphoid tissues such as lungs (alveolar macrophages), liver (Kupffer cells) and skin (Langerhans cells). Considering malignancies, IL2RA protein is expressed in many types of neoplastic cells, such as in most B-cell neoplasms, T-cell lymphomas, some acute nonlymphocytic leukemias, neuroblastomas, mastocytosis, Waldenstrom macroglobuliaemia and tumor infiltrating lymphocytes. Structure Interleukin-2 receptor alpha chain is an integral-membrane protein, more precisely Type I transmembrane protein. This bitopic polypeptide is constructed by a sequence of 272 amino acids and has a molecular mass of around 30.8 kDa.