Myotonic dystrophy (DM) is a dominantly inherited neuromuscular disease, highly variable and multisystemic, which is caused by the expansion of a CTG repeat located in the 3' untranslated region of the DMPK gene. Normal alleles show a copy number of 5-37 repeats on normal chromosomes, amplified to 50-3000 copies on DM chromosomes. The trinucleotide repeat shows a trimodal allele distribution in the majority of the examined population. The first class includes alleles carrying (CTG)5, the second class, alleles in the range 7-18 repeats, and the third class, alleles (CTG) > or =19. The frequency of this third class is directly related to the prevalence of DM in different populations, suggesting that normal large-sized alleles predispose toward DM. We studied CTG repeat allele distribution and Alu insertion and/or deletion polymorphism at the myotonic dystrophy locus in two major Ethiopian populations, the Amhara and Oromo. CTG allele distribution and haplotype analysis on a total of 224 normal chromosomes showed significant differences between the two ethnic groups. These differences have a bearing on the out-of-Africa hypothesis for the origin of the DM mutation. In addition, (CTG) > or =19 were exclusively detected in the Amhara population, confirming the predisposing role of these alleles compared with the DM expansion-mutation.