MBNL1 and myotonic dystrophy type 1: In DM type 1 (DM1), the transcription of a CTG microsatellite expansion (CTGexp) in the 3′ untranslated region (3′ UTR) of the DMPK gene results in the expression of toxic CUGexp RNA, which sequesters muscleblind-like (MBNL) proteins and blocks their splicing activity (2, 3).