IL-9 is produced by Th9 cells which are a subset of Th2 cells expressing peroxisome proliferator-activated receptor gamma [25], which develop in the presence of transforming growth factor-beta (TGF-β) and IL-4 [26,27,28]; IL-9 plays a role in allergy development [29,30,31,32,33] by increasing IgE expression on B cells [34] and causing bronchial hyperresponsiveness in inhaled methacholine-induced asthma [14,35]. The gene discussed is IL4; the disease is asthma.