The (CTG)n array is embedded within a large CpG island of 3.5 kb in size, which encompasses a complex genomic region covering the 3′ end of the DMPK gene, the DM1-AS gene, and the 5′ end of the downstream SIX5 gene (Figure 1). The gene discussed is DMPK; the disease is myotonic dystrophy type 1.