DM1 is caused by a CTG repeat expansion in the DMPK gene, which results in a toxic gain-of-function mRNA that sequesters MBNL1, an ubiquitously expressed splicing factor, in the nucleus resulting in disordered MBNL1 autoregulation of its pre-mRNA splicing. The gene discussed is DMPK; the disease is myotonic dystrophy type 1.