CD4 T cells play a central role in the disease process by producing Th2 effector cytokines (e.g. IL-4, IL-5, IL-9, IL-13) that drive many of the hallmark phenotypic changes observed in asthma; these include airways hyperresponsiveness, increased mucus production, mucus cell hyperplasia, and eosinophilic inflammation. This evidence concerns the gene IL4 and asthma.