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Sin a I, a 2-S albumin from the seeds of yellow mustard, is herein described as the major allergen of these seeds. This protein is composed of two disulfide-linked polypeptide chains of 39 and 88 amino acids, whose primary structures are reported. The Sin a I allergen is found to be related to other low-molecular-mass albumins, such as those isolated from rapeseed, castor bean and Brazil nut. Additional structural similarity has also been found between the glutamine-rich large chain of Sin a I and a proline-rich zein, a gliadin, and trypsin and alpha-amylase inhibitors isolated from the seeds of several monocotyledons. Internal amino acid sequence similarity has been detected at both termini of the small and large chains of Sin a I and involves the location of proline and glycine residues at similar positions in relation to the processing cleavage sites. Prediction of secondary structure, based on the amino acid sequences of the mature chains of the mustard allergen, indicates that the precursor polypeptide is cleaved at regions showing a high beta-turn probability. This is also observed with the amino acid sequence deduced from the rapeseed napin gene nucleotide sequence.