Pollen from cedar and cypress trees is a major cause of seasonal hypersensitivity in humans in several regions of the Northern Hemisphere. Jun a 1 is a cedar allergen from the pollen of the mountain cedar Juniperus ashei (Cupressaceae). The core of the structure consists primarily of a parallel beta-helix, which is nearly identical to that found in the pectin/pectate lyases from several plant pathogenic microorganisms. Four IgE epitopes mapped to the surface of the protein are accessible to the solvent. The conserved vWiDH sequence is covered by the first 30 residues of the N terminus. The potential reactive arginine, analogous to the pectin/pectate lyase reaction site, is accessible to the solvent, but the substrate binding groove is blocked by a histidine-aspartate salt bridge, a glutamine, and an alpha-helix, all of which are unique to Jun a 1.