This is a phase 1/2a double-blinded, placebo-controlled, dose-escalating study to evaluate the safety, tolerability, pharmacokinetics (PK) and pharmacodynamics (PD) of single and multiple ascending doses of ARO-DM1 compared to placebo in male and female subjects with type 1 myotonic dystrophy (DM1). Participants who have provided written informed consent and met all protocol eligibility requirements will be randomized to receive single (Part 1) or multiple (Part 2) doses of ARO-DM1 or placebo.