Study of ATX-01 in Participants With DM1

Key Inclusion Criteria:

• Participants with a documented clinical diagnosis of DM1 (CTG expansion of >150repeats in DMPK gene measured in peripheral blood mononuclear cells)

• Ambulatory, defined as able to complete a 10-meter walk/run test at screeningwithout the use of assistive devices such as canes, walkers, or orthoses, except forankle-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 orsignificant 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