The specific high affinity functional IL-9 receptor complex contains the common gamma chain as well as the IL-9 receptor (IL-9 R). The mouse and human transmembrane proteins share 53% amino acid sequence identity. As a result of alternate splicing, cDNA clones encoding isoforms of IL-9 R, including a putative soluble form, have also been identified. Cells known to express IL-9 R include T cells, neutrophils, mast cells and macrophages.