In the repeat expansion disease myotonic dystrophy (dystrophia myotonica, DM), expanded CTG or CCTG repeats are transcribed into toxic RNA that form intranuclear foci, sequestering and functionally depleting the Muscleblind-like (MBNL) family of RBPs19. The gene discussed is MBNL1; the disease is myotonic dystrophy type 1.