MBNL1 and myotonic dystrophy type 1: DMX<sup>on</sup> controlled expression of therapeutic MBNL1 protein can improve skeletal muscle myotonia or prevent cardiac toxicity due to MBNL1 overexpression in mice.<h4>Interpretation</h4>DMX<sup>on</sup> control elements can increase the therapeutic window of viral-based therapeutics in DM1, and activity is dependent upon delivered cargo and model severity.