Myotonic dystrophy (DM) is a disorder of progressive muscle weakness and wasting caused by repeat expansion in genes encoding DM1 or DM2. The mutated protein becomes trapped in the nuclei of mutant cells, where it associates with RNA-binding factors, including muscleblind-like (MBNL) proteins, disrupting splicing. Sequestration of MBNL proteins is thought to be central to the pathology of DM. Machuca-Tzili et al. explore this hypothesis using a zebrafish model and show that deficiency for mbnl2 recapitulates several features of human DM. The finding that mbnl2 is also important for zebrafish brain development opens up new questions about cognitive function in DM patients.