CTCF and myotonic dystrophy type 1: CpG methylation around the repeats and binding with the insulator protein CTCF could regulate CTG repeat instability.25 In particular, aberrant CpG methylation at CTCF binding site upstream of the repeats is associated with repeat length in CDM muscles.33 Our results also indicate a possible correlation between methylation status and repeat instability in CNS cells of DM1 (i.e., greater repeat instability in cortical neurons and less instability in spinal neurons).