Study of ARO-DM1 in Subjects With Type 1 Myotonic Dystrophy

Inclusion Criteria:

• Genetically confirmed diagnosis of DM1

• Clinician-assessed signs of DM1 including clinically apparent myotonia

• Onset of DM1 symptoms occurred after the age of 12 years

• Walk for at least 10 meters independently at Screening

• Subjects of childbearing potential must agree to use highly effective contraceptionin addition to a condom during the study and for at least 90 days following the endof study or last dose of study drug, whichever is later. Subjects must not donatesperm or eggs during the study and for at least 90 days following the end of studyor last dose of study drug whichever is later.

Exclusion Criteria:

• Inadequately controlled diabetes

• Confirmed diagnosis of congenital DM1

• Uncontrolled hypertension

• History of thromboembolic events

• History of Tibialis Anterior (TA) biopsy within 3 months of Day 1 or planning toundergo TA biopsies during the study period

• Clinically significant cardiac, liver or renal disease

• HIV infection (seropositive) at Screening

• Seropositive for hepatitis B (HBV) or hepatitis C (HCV) at screening

• Untreated or poorly controlled epilepsy

• Treatment with anti-myotonia medication within a period of 5 half-lives of themedication prior to Screening.

• Abnormal coagulation parameters at Screening including platelet count, InternationalNormalized Ratio (INR), prothrombin time, and activated partial thromboplastin time (APTT)

• History or presence of any of the following: hypercoagulable state, nephrotic rangeproteinuria, antiphospholipid antibody syndrome or myeloproliferative diseases,inability to ambulate, use of hormone-based contraceptives and peri/post- menopausalhormone replacement therapy ≤ 16 weeks prior to Day 1

Note: Additional inclusion/exclusion criteria may apply per protocol