Background: We have previously shown that the allergic sensitization to ovalbumin does not represent a superantigen-like immune response. In gene-targeted mice (Delta D-iD) with a single modified Diversity gene segment (D(H)) of the immunoglobulin heavy chain, enriched for charged amino acids, the asthma phenotype in a murine model was markedly alleviated compared to wild-type animals. Objective: We now sought to determine whether the confinement to a single D(H) gene segment alone leads to a reduced allergic phenotype. Methods: We examined another gene-targeted mouse strain (Delta D-DFL) with a single D(H) gene segment which encodes for neutral amino acids, thus reflecting the preferential repertoire in wild-type mice. Mice were sensitized intraperitoneally to ovalbumin. Results: Despite the constraint to a single D(H) gene segment, Delta D-DFL mice mounted high total and allergen-specific IgG(1) and IgE serum levels after sensitization to ovalbumin. The affinity constants of allergen-specific IgG(1) antibodies did not differ between Delta D-DFL and wild type. Following challenge with aerosolized allergen, a marked local T(H)2 cytokine response and an eosinophilic airway inflammation developed. Quantitative histology revealed increased mucus production and intense goblet cell metaplasia which were identical to those in wild type. Moreover, Delta D-DFL mice developed an airway hyperreactivity to methacholine and to the specific allergen, which both did not differ from those in wild-type animals. Conclusion: A single D(H) gene segment is sufficient for the establishment of the asthma phenotype in a murine model of allergic airway inflammation. Thus, the allergic phenotype depends on the amino acid composition and not on the diversity of the classical antigen-binding site.