For IL-4 and IL-13, the unique utilization of the STAT6 transcription factor in the signaling they elicit allows them to execute specific functions on different cell types; IL-4 is the regulator of lymphocyte functions (Th2 differentiation and B-cell IgG1 and IgE class switch), whereas IL-13 is an effector cytokine, regulating smooth cell muscle contraction and mucus production in the airway epithelium, for example, in allergic asthma (15). The gene discussed is IL13; the disease is allergic asthma.