Myotonic dystrophy: the correlation of (CTG) repeat length in leucocytes with age at onset is significant only for patients with small expansions

Myotonic dystrophy (DM) was the first of a group of diseases to be identifi ed for which the genetic basis is the expansion of a triplet repeat. Myoton ic dystrophy also exhibits anticipation, in which the disease worsens throu gh successive generations. These two features have led many groups to analy se whether a significant negative correlation between triplet repeat length and severity of disease exists. However, the recent molecular finding that two distinct subsets of classically affected DIM patients exist, those who export expansion derived DMPK RNA and those who do not, led us to question whether combining data from these two sets of patients is statistically va lid. We found that although patients with small expansions showed a signifi cant correlation between age at onset and triplet repeat length, those with larger expansions did not. The theoretical triplet repeat size, which sepa rated the two groups, was also deduced.