Nterleukin-9 (IL-9) was originally identified in mice as a T cell growth factor and is a member of the common γ-chain-receptor cytokine family, with other members including IL-2, IL-4, IL-7, IL-15 and IL-21. The IL-9 gene loci of both humans and mice have a similar organization, consisting of five exons, and share a 55% amino acid homology at the protein level. The IL-9 receptor consists of the cytokine-specific IL 9 receptor α-chain (IL-9Rα) and the γ-chain1. IL-9 induced receptor activation promotes the cross phosphorylation of Janus kinase 1 (JAK1) and JAK3. This cross-phosphorylation leads to the downstream activation of signal transducer and activator of transcription (STAT) complexes, specifically STAT1 homodimers, STAT5 homodimers and STAT1–STAT3 heterodimers. Although the contribution of IL-9 to the activation of these pathways in primary cells has not been fully elucidated, it may be of relevance in vivo.