MBNL1 and myotonic dystrophy type 1: Substantial evidence that CUGexp RNAs are toxic comes from studies of the neuromuscular disease myotonic dystrophy (DM) where CUGexp (DM1) and CCUGexp (DM2) RNAs alter the activities of two families of alternative splicing factors, the MBNL and CELF proteins [5],[6].