IL9 encodes the human gene, Interleukin 9. It may also be known as: Interleukin-9; IL-9; Cytokine P40; T-cell growth factor P40; p40 T-cell and mast cell growth factor; p40 cytokine; homolog of mouse T cell and mast cell growth factor 40; HP40. The encoded protein has an amino acid length of 144 and a mass of 15.9 kDa. IL9 is a member of the IL-7/IL-9 family. Homologs of this gene exist in other organisms, including: Frog.