Multisystem manifestations in myotonic dystrophy type 1 (DM1) may be due to dosage reduction in multiple genes induced by aberrant expansion of CTG repeats in DMPK, including DMPK, its neighboring genes (SIX5 or DMWD) and downstream MBNL1. However, direct evidence is lacking. This evidence concerns the gene MBNL1 and myotonic dystrophy type 1.