MBNL1 and myotonic dystrophy type 1: A series of studies traced the cause of myotonic dystrophytype1 (DM1) to the presence of an expanded trinucleotide repeat expansion,r­(CUG)exp, in the 3′ UTR of dystrophia myotonicaprotein kinase (DMPK) mRNA.−, , , , , , ,  When repeat length exceeds >50 repeats, the RNA folds upon itselfforming a periodic array of 1 × 1 nucleotide UU internal loopsthat are high affinity binding sites for muscleblind-like 1 (MBNL1)and other RBPs (Figure ).