CELF1 and myotonic dystrophy type 1: DM1 is caused by a genetic defect involving the expansion of a CTG repeat in the 3′ untranslated region (UTR) of DMPK at 19q13.3, resulting in transcription of toxic RNAs that contain an expanded CUG repeat.2 In turn, toxic RNAs alter the activity of RNA binding proteins involved in alternative splicing, such as muscleblind-like splicing regulator (MBNL) and CUGBP Elav-like family member 1 (CELF1), eventually perturbing the regulation of pre-mRNA splicing in affected tissues.