The primary driver of DM1 is RNA toxic gain-of-function, where transcription of the expanded allele produces long CUG-RNA repeats (rCUGexp) that form imperfect hairpin structures (4, –6), sequestering muscleblind-like protein 1 (MBNL1) and other RNA splicing regulators (2). The gene discussed is MBNL1; the disease is myotonic dystrophy type 1.