The IL-2 receptor system consists of three non-covalently linked subunits termed IL-2Rα, IL-2Rβ, and IL-2Rγ. The IL-2Rα is a type I transmembrane protein consisting of a 219 amino acid extracellular domain, a 19 amino acid transmembrane domain and a 13 amino acid intracellular domain, which is not involved in the transduction of IL-2 signals. Proteolytic processing of IL-2Rα releases the entire extracellular domain of IL-2Rα, thereby generating a 219 amino acid soluble protein called soluble IL-2Rα (sIL-2Rα). The homodimeric form binds IL-2 (KD=10mM) and facilitates IL-2 signaling. The secreted sIL-2Rα is expressed on leukemia cells, lymphoma cells, and newly activated T and B cells, as well as on approximately 10% of NK cells. Recombinant Human sIL-2Rα is a 24.8 kDa protein containing 219 amino acid residues consisting of only the extracellular domain of IL-2Rα. Due to glycosylation, IL-2Rα has an approximate molecular weight of 31 kDa based on SDS-PAGE gel and Mass Spectrometry.