Cerebellar degeneration-related antigen (designated CDR34) was previously cloned by antibody screening of a cDNA library and was shown to be one of the target molecules recognized by autoantibodies in patients with paraneoplastic cerebellar degeneration. This molecule is distinctive in that it contains a tandem hexapeptide repetitive structure, presumably the basis for its high immunogenicity. In this study, we cloned the human CDR34 gene and proved that the entire repetitive sequence is encoded by a single exon without introns. We also showed that the nucleotide repeats are preserved only in the protein-coding sequences, suggesting evolutionary constraint in this region of the gene. Corresponding mouse cDNA clones were also isolated, which encoded a larger molecule with very similar hexapeptide repeating units. Comparison of the human and mouse repeats revealed a highly conserved Glu-Asp core in each unit, implicating the functional significance of this motif. Chromosomal mapping by somatic cell hybrid analysis mapped CDR34 to both human and mouse chromosomes X, and in situ hybridization further assigned CDR34 to human Xq24-q27.