The purpose of this study is to assess the safety and tolerability of ARGX-119 in adult participants with DOK7- Congenital Myasthenic Syndromes. The study will also assess how ARGX-119 is processed by the body (pharmacokinetics), how the immune system reacts to it (immunogenicity), and how it may improve the way patients feel and function.

After the screening period, eligible participants will be randomized in a 4:1 ratio to receive intravenous infusions of ARGX-119 or placebo during the double-blinded treatment period. Participants will then enter the follow-up period. After the follow-up period, participants may enrol in the active-treatment period, where they will receive open-label ARGX-119.

The full duration of the study is approximately 38 months.

Inclusion Criteria:

• At least 18 years of age.
• Has genetically confirmed congenital myasthenic syndromes due to mutation of downstream of kinase 7 (DOK7-CMS).
• Participants taking oral beta agonists (eg, albuterol, salbutamol, ephedrine) must have been receiving the medication for more than 3 months and agree to remain on a same stable dosing regimen of the same medication until the end of the study.

Exclusion Criteria:

• Diagnosis of CMS due to mutation of any gene other than DOK7.
• Known medical condition that would interfere with an accurate assessment of CMS, confound the results of the study, or put the patient at undue risk, as assessed by the investigator.
• History of malignancy, cancer, unless considered cured by adequate treatment with no evidence of recurrence for more than 5 years. Adequately treated participants with the following cancers can be included at any time: Basal cell or squamous cell skin cancer, Carcinoma in situ of the cervix, Carcinoma in situ of the breast, Incidental histological findings of prostate cancer.
• Pregnant or lactating state or intention to become pregnant during the study.