This study is testing a drug called zeleciment basivarsen (DYNE-101) to see if it helps treat myotonic dystrophy type 1 (DM1), a condition that causes muscle problems. The study has three parts. First, you will have a Screening period that lasts up to 8 weeks to see if you qualify. Then, you will be part of a Placebo-Controlled Period for 48 weeks, where you might receive the study drug or a placebo (a "fake" drug with no effect). Finally, there's a Long-Term Extension Period of 24 weeks to see how you do over time. A special group of experts will regularly check the safety of the study.

• The study lasts for more than a year, with different periods.
• You must be able to walk and stand with some help.
• There are specific health rules to join, like not having had recent major surgery.