Our results indicate that daunorubicin directly interacts with expanded CUG RNA repeats and impairs binding of MBNL1 to these repeats, thus promoting MBNL1 release from CUG RNA foci in DM1 cells, as well as in DM1 Drosophila cardiomyocytes. Here, MBNL1 is linked to myotonic dystrophy type 1.