We read with interest the article by Wheeler et al, who reported on factors associated with mutant CAG repeat instability in Huntington's disease (HD).1 Familial clustering appeared to be one of the factors involved, as repeat instability was found to be correlated between siblings (r = 0.28).1 Surprisingly, however, the authors did not report on a far more sensitive and direct measure of heritability—that is, the relationship between CAG repeat-length changes upon inheritance and …