A Study to Evaluate RO7204239 in Participants With Facioscapulohumeral Muscular Dystrophy

Inclusion Criteria: * Genetic confirmation of FSHD1 or FSHD2 * Clinical findings consistent with FSHD * Ability to walk unassisted * Ricci Clinical Severity Scale score ≥ 2.5 and ≤ 4 * Agreement to maintain the same frequency and intensity of physiotherapy, occupational therapy, and other forms of exercise during the clinical study Exclusion Criteria: * Pregnancy or breastfeeding, or intention of becoming pregnant during the study or within 17 months after the final dose of RO7204239 * Current or previous treatment (or receipt) of anti-myostatin therapies * Treatment with any investigational therapy within 90 days prior to screening, or 5 drug-elimination half-lives of the drug, whichever is longer * Contraindications to MRI scans * Presence of clinically significant ECG abnormalities * Presence of clinically significant cardiovascular disease * Presence of clinically significant abnormal findings in echocardiography at screening, with the exception of mitral valve prolapse, which does not exclude participants from the study * Any major illness within 1 month before screening * Ascertained or presumptive hypersensitivity (e.g., anaphylactic reaction) to RO7204239, or to the constituents of its formulation * History of malignancy (except in situ basal cell carcinoma of the skin and in situ carcinoma of the cervix of the uterus that have been excised and resolved with documented clean margins on pathology) * Any clinically relevant history of anaphylactic reaction requiring inotropic support * Any abnormal skin conditions, pigmentation, or lesions in the area intended for SC injection (abdomen) and that would prevent visualization of potential injection-site reactions to RO7204239 * Immobilization, surgical procedures, fracture, or trauma to the upper or lower limbs within 90 days prior to screening or longer, if judged by the investigator that it may affect motor function assessment * Any planned surgery that may affect a participant's motor function assessment, including participants who have had surgery of scapular fixation within the 12 months preceding screening or that are planned during the study * Use of the following medications within 90 days prior to enrollment: salbutamol or another β2-adrenergic agonist taken orally; creatine; recombinant human growth hormone; recombinant human insulin growth factor-1; testosterone, oxandrolone, or other anabolic steroid; chronic oral or parenteral use of corticosteroids (inhaled corticosteroid use is allowed) unless required to manage injection reactions;agents anticipated to increase or decrease muscle volume or strength