Both IL-4 and IL-13 induce IgE synthesis in B cells by binding to their functional receptors on target cells. These receptors are considered to be composed of heterodimers and both share the IL-4R alpha chain (IL-4R alpha) as a component. Atopy is an inherited tendency, underlying asthma, rhinitis and eczema, and generating high nonspecific IgE and/or high specific IgE against common antigens. Based on findings concerning the molecular mechanism of the signal transduction of IL-4 and IL-13, IL-4R alpha was considered a gene that gave rise to atopy. One polymorphism in the IL-4R alpha gene, Ile50Val, has been correlated with atopy by both genetic and functional assessment. The strategy used in these studies should lead to identification of other genes involved in atopy. Furthermore, these studies should be useful for gene diagnosis of atopy and development of new therapies for atopy in the future.