Several studies have associated elevated levels of house dust endotoxins with a) increased respiratory symptoms in infants (Park et al. 2001a); b) worsening of existing asthma that is independent of the levels of other common indoor allergens (Michel et al. 1991, 1996); c) decreased frequency of positive IgE-mediated skin test reactions in infants (Gereda et al. 2000b); and d) decreased occurrence of hay fever and positive prick skin test in children (Braun-Fahrlander et al. 2002). This evidence concerns the gene IGHE and asthma.