Global Study of Del-desiran for the Treatment of DM1

Key Inclusion Criteria:

• Clinical and genetic diagnosis (CTG repeat ≥ 100) of DM1

• Ability to walk independently (orthoses and ankle braces allowed) for at least 10meters at screening

Key Exclusion Criteria:

• Breastfeeding, pregnancy, or intent to become pregnant during the study

• Unwilling or unable to comply with contraceptive requirements

• Abnormal lab values, conditions or diseases that would make the participantunsuitable for the study

• Diabetes that is not adequately controlled

• History of decompensated heart failure within 3 months of screening. Participantswith preexisting pacemaker/ICD are not excluded.

• Body Mass Index > 35 kg/m2 at Screening

• Recently treated with an investigational drug or biological agent

• Treatment with anti-myotonic medication within 5 half-lives or 14 days of baseline,whichever is longer, prior to baseline.

Note: Additional protocol defined Inclusion and Exclusion criteria apply