The primary structure of mouse SVS I was determined by peptide sequencing and nucleotide sequencing of cloned cDNA. The precursor molecule consists of 820 amino acid residues, including a signal peptide of 24 residues, and the mature polypeptide chain of 91 kDa has one site for potential N-linked glycosylation. The SVS I is homologous with amiloride-binding protein 1 (ABP1), a diamine oxidase. However, it probably lacks enzymatic activity, because the cDNA codes for His instead of Tyr at the position of the active-site topaquinon. The SVS I monomer probably binds one molecule of copper, because the His residues coordinated by Cu(II) are conserved. The SVS I gene consists of five exons and is situated on mouse chromosome 6,B2.3. It is located in a region of 100 kilobases (kb) containing several genes with homology to SVS I, including the gene of ABP1 and two other proteins with homology to diamine oxidase. The locus is conserved on rat chromosome 4q24, but the homologous region on human chromosome 7q34-q36 solely contains ABP1. The other genes with homology to diamine oxidase were probably present in a progenitor of primates and rodents but were lost in the evolutionary lineage leading to humans—presumably during recombination between chromosomes. The estimated molecular mass of rat SVS I is 102 kDa (excluding glycosylation). The species difference in size of SVS I is caused by tandem repeats of 18 amino acid residues in the central part of the molecule: The mouse has seven repeats, and the rat has 12 repeats.