⁃ Subjects who do not enter this study directly from completing the AMO-02-MD-2-003 study (i.e. subjects who did not complete AMO-02-MD-2-003, subjects who completed AMO-02-MD-2-003 but did not directly rollover or subjects who are re-entering AMO-02-MD-2-004), will not be considered eligible for the study without meeting all of the criteria below:

• Subjects under study must be individuals with a diagnosis of Congenital or Childhood Onset DM1.

• Diagnosis must be genetically confirmed

• Subjects must be male or female aged ≥6 years to ≤45 years at Screening

• Subjects must have a Clinical Global Impression - Severity (CGI-S) score of 3 or greater at Screening (V-1)

• Written, voluntary informed consent must be obtained before any study related procedures are conducted. Where a parent or legally authorized representative (LAR) provides consent, there must also be assent from the subject (as required by local regulations)

• Subject's caregiver must be willing and able to support participation for duration of study

• Subject must be willing and able to comply with the required food intake restrictions as outlined per protocol

⁃ Subjects entering directly from completing the antecedent AMO-02-MD-2-003 study will not be considered eligible for the study without meeting all of the criteria below:

• Subjects who have completed the antecedent AMO-02-MD-2-003 study through V11

• Written, voluntary informed consent must be obtained before any study related procedures are conducted. Where a parent or LAR provides consent, there must also be assent from the subject (as required by local regulations)

• Subject's caregiver must be willing and able to support participation for duration of study

• Subject must be willing and able to comply with the required food intake restrictions as outlined per protocol