IL-25 (also known as IL- 17E), a member of the IL-17 family, has been implicated in Th2 cell-mediated immunity. Recently, IL-25 has been identified as one of the initiators of the Th2 response (10, 11) and has also been shown to be expressed by mast cells. The transgenic overexpression of IL-25 by lung epithelial cells results in mucus hyperproduction and airway infiltration of macrophages and eosinophils; conversely, IL-25 blockade reduced airway inflammation and Th2 cytokine production in an allergen-induced asthma model.