This study is checking how well a new medicine called zeleciment basivarsen (DYNE-101) works for people with myotonic dystrophy type 1 (DM1), a muscle and genetic condition. It will also look at how safe and easy it is to use. The study has three parts: a Screening period (up to 8 weeks), a Placebo-Controlled Period (48 weeks), and a Long-Term Extension Period (24 weeks). A group of experts will regularly review the safety of participants. To join, you need to be able to walk a short distance independently and have a specific genetic test result.

• The study lasts about a year and a half, including all three periods.
• Participants must not have had major surgeries recently, except for certain devices.
• Some medications are not allowed before the study starts, like GLP-1 agonists.

Think about the study's length, any medication restrictions, and if you meet the health requirements before deciding to participate.