Most asthma of childhood onset and a significant proportion of asthma of later onset is allergic, characterised by accumulation in the airway mucosa of activated CD4+ T-lymphocytes with a Th2 pattern of cytokine secretion i.e. predominantly interleukin (IL) -4, IL-5 and IL-13; mast cells and macrophages, notably within the airway epithelium; and especially during an acute attack, recruitment of numerous eosinophils [2], [3]. This evidence concerns the gene IL13 and asthma.