MBNL1 and myotonic dystrophy type 1: At the molecular level, DMPK transcripts containing CUG expansions form toxic RNA foci which sequester splicing factors such as muscleblind-like (MBNL) proteins, while also increasing levels of CUG-binding protein 1 (CELF1), leading to altered RNA splicing events causing DM1-related symptoms (Masuda et al., 2012; Wang et al., 2012).