Until recently the CTG array in the DMPK gene was assumed to be a pure tract (without interruptions/variant repeats), in contrast to a majority of other simple DNA repeats associated with “disorders of unstable repeat expansion.” However, 4-5% DM1 individuals carry interrupted expanded alleles with interruptions being multiple CCG triplets, CCGCTG hexamers or CTC triplets, all located at the 3′ end of the CTG array [43, 44]. Here, DMPK is linked to myotonic dystrophy type 1.