Study of ATX-01 in Participants With DM1

Key Inclusion Criteria: * Participants with a documented clinical diagnosis of DM1 (CTG expansion of \>150 repeats in DMPK gene measured in peripheral blood mononuclear cells) * Ambulatory, defined as able to complete a 10-meter walk/run test at screening without the use of assistive devices such as canes, walkers, or orthoses, except for ankle-foot orthoses * Presence for \>3 seconds of grip myotonia as confirmed by a central reader Key Exclusion Criteria: * Participants with congenital DM1 * Medical Research Council Muscle Scale score of less than 4 on ankle dorsiflexion or significant tibialis anterior atrophy that prevents a muscle biopsy * Use of mexiletine or other agent for myotonia within 21 days or 5 half-lives, whichever is longer, prior to screening