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BACKGROUND:A number of allergenic proteins in animal danders have been characterized at the molecular level, but little is known of their biologic functions. We have found that the prevalence of IgE antibodies among patients with cattle-associated asthma is highest against a dander protein referred to as BDA20. OBJECTIVE:The study was performed to characterize the molecular structure of BDA20,* the predominant allergen in bovine dander. METHODS:Clones encoding allergens were identified and isolated from a complementary DNA library by immunoblotting and DNA hybridization and sequenced. Recombinant proteins were produced in Escherichia coli.Immunoreactivity of the recombinant proteins and amino acid sequences of peptides obtained from native BDA20 after Lys-C cleavage were used to identify clones coding for BDA20. RESULTS:In this article we report the cDNA and amino acid sequences of BDA20. Homology comparisons showed that BDA20 belongs to the family of lipocalins. CONCLUSIONS:The results link a dander allergen to a group of functionally important proteins. Lipocalins are present in various body fluids and secretions of several animal species in which they function as carriers of small hydrophobic molecules, such as retinoids and pheromones. If allergenicity proves to be a property shared by lipocalins, our results will have considerable implications for allergen research. (J ALLERGYCLINIMMUNOL1996;97:1297-1303.)