HDL2 has thus far been identified in about 1% of individuals with an HD-like disorder who do not have HD. The actual percentage, at least in the U.S., is probably somewhat higher if analysis is restricted to individuals with a family history and a clear HD-like presentation, with exclusion of individuals who likely have tardive dyskinesia or a tic disorder. So far, all individuals with HDL2 have either definite or probable African ancestry. In South Africa, preliminary analysis suggests that HDL2 is almost as common as HD in individuals of African descent . It therefore appears that the HDL2 expansion has an African origin. The distribution of HDL2, and the length of the normal allele in various African populations, remains to be determined.
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Repeat Length (triplets)
Fig. 4 HDL2 repeat length is correlated with an earlier onset age. N = 24, R = 0.62, r2 = 0.39, p = 0.0011. (Reprinted, with permission, from Margolis et al. .)
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