To confirm that the nickase treatment specifically targeted the extreme DM1 repeat expansion only, well-known stable, non-pathogenic CNG-rich repeats in AR, ATN1, ATXN1, PPP2R2B, TBP, and TCF4 were screened for off-target activity by PCR amplification and Sanger sequencing and aligned against the background of the iDM2900 cell line (Table S4). This evidence concerns the gene AR and myotonic dystrophy type 1.