It has been shown that expansion of the CTG repeats in DMPK produces allele-specific effects on transcription of the two adjacent genes, SIX5 (downstream of DMPK) and DMWD (upstream of DMPK) through changes of the local chromatin structure, leading to down-regulation of SIX5 and DMWD in DM1 patients.14,18–21,23,47–51 The effects of SIX5 in DM1 have been well characterized using Six5-knockout mice;35,36,52 however, the role of DMWD in DM1 has not been analyzed yet. Here, DMPK is linked to myotonic dystrophy type 1.