Myotonic dystrophy (DM) is associated with abnormal expansions of the CTG 
repeats in the 3' untranslated region of its gene. Previous studies in 
individuals of European origin demonstrated strong linkage disequilibrium 
between different CTG repeat length alleles and an Alu element 
insertion/deletion polymorphism in intron 8 of the DM gene: CTG11-13 chromosomes 
were almost exclusively associated with the deletion allele, while chromosomes 
with five or 19-30 repeats or disease chromosomes were only found on the 
insertion allele. One of the models suggested by these results proposed that the 
triplet repeats on insertion-associated chromosomes were particularly prone to 
mutation. Studies of other triplet repeat disorders have suggested that arrays 
of perfect repeats are more prone to instability than those that are 
interrupted. We have examined the evolution of this locus by typing a variety of 
primates and samples drawn from several different human populations for both CTG 
repeat length and the insertion/deletion polymorphism. DM gene sequences from 
different primates revealed inter- and intraspecies variability in the number of 
CTG repeats. Human chromosomes with five repeats or 11-13 repeats (the two major 
modes of the human CTG repeat distributions) showed no evidence of preferential 
stabilization of these repeat sizes by imperfect sequences. The insertion and 
deletion allele frequencies showed large interpopulation variation and the 
degree of association between these alleles and various CTG repeat lengths is 
not nearly as complete as was previously supposed. We have found deletion 
alleles carrying long normal (> 19) CTG repeats and insertion alleles associated 
with CTG11 - 13 in two african populations.(ABSTRACT TRUNCATED AT 250 WORDS)