The frequencies of haplotypes based upon the
(CTG)n repeat and three other biallelic markers in and
around the myotonic dystrophy (DM) locus were estimated
in 13 ethnically, linguistically and geographically
diverse sub-populations of India. The range of CTG repeats
in caste populations was 5–31, while in tribal populations
the range was shorter (5–23). Extensive variation
in frequencies of large (CTG)n alleles (≥18 repeats) was
found in Indian populations. The implications of this finding
on DM epidemiology are discussed. Haplotype diversity
was found to be very high in most populations. The
majority of the Indian DM patients carried a haplotype
that is commonly found among DM patients globally; this
is the most common haplotype in the class of large (≥18
repeats) CTG alleles. However, one haplotype was found
to be present in particularly high frequency in Indian populations; this haplotype was also found among Indian DM
patients. This haplotype may be a characteristic of Indian
and possibly of other East Asian populations