Long tandem dinucleotide repeats composed of alternating purines and pyrimidines [RY(i)] are abundant and highly polymorphic. "Simple" RY(i) are predominantly composed of one tandem repeat of a dinucleotide sequence. In contrast, "cryptic" RY(i) [cRY(i)] are composed of multiple short dinucleotide repeats. Herein, we describe the racial distribution of alleles for a polymorphic cRY(i) in the factor IX gene. Allele I is absent in Asians, whereas allele III is rare or absent in Caucasians or blacks. A polymorphic cRY(i) analyzed previously shows even more dramatic variation among racial groups, hinting that a battery of cRY(i) might have utility in assessing the racial origin of a DNA sample.