DMPK and myotonic dystrophy type 1: Although the majority of patients with DM1 likely inherit a pure CTG repeat expansion at the DMPK locus, it is known that a subset of patients contains variant repeat interruptions within the disease allele.31,35 Such variant repeats, most commonly CCG and CGG, have been shown to reduce both germline and somatic instability31 and appear to be associated with less severe symptoms.31,35, –, 37 Thus, we used SP-PCR to amplify the CTG repeat tract and test for the presence of variant repeats by post-PCR digestion with AciI (figure 1B).