TH2-cell derived cytokines such as interleukin 4 (IL-4) promote B cell isotype switching to immunoglobulin E (IgE) which plays a key role in allergic asthma by acting as a link between an allergen and the mast cell to which the IgE is attached by its constant domain to membrane-bound Fcε receptors5–7. The gene discussed is IL4; the disease is allergic asthma.