Interleukin-9 (IL-9) is expressed primarily by activated T cells. The growth factor and anti apoptotic activities of IL-9 on multiple transformed cells suggest a potential role for the cytokine in tumourigenesis. IL-9 has also been proposed as a candidate gene for asthma IL-9 is a Th2-derived cyto kine that has been reported to regulate T and B cell function. IL-9was first isolated as a factor capable of sustaining the long-termgrowth of murine T cell clones, but has since been demon strated to have in vitro activities on mast cells, erythroid pro genitors, and B cell Ig expression.