The FLEX study is a randomized, blinded, and placebo-controlled study assessing the efficacy and safety of an investigational drug for adults diagnosed with Generalized Myasthenia Gravis (gGM). The injectable investigational drug is being evaluated over a 12-week induction period and a 12-week maintenance period. Based on completion of these periods and a participant's response to the investigational injection, participants may qualify to continue in a 52-week extension study of the investigational drug. If you choose to enter this study, you will participate in the study for up to approximately 84 weeks (1 ½ years) and you will be asked to attend up to 34 study visits; visits can last from 1 to 4 hours.

It is unknown at this time whether the study treatment will improve participant's health. The potential risks of this study include the possibility of infections, low albumin levels, and increases in blood cholesterol.

The purpose of this study is to see if nipocalimab is safe and useful for treating Adults with Generalized Myasthenia Gravis. Myasthenia gravis (MG) is a rare disease that affects the muscles and brain; causes tiredness and weakness in the eye muscles, arms, legs, and neck, and may cause difficulty with breathing. You must be diagnosed with myasthenia gravis to be in this study. Another purpose of this study is to find out how long nipocalimab stays in and acts on your body. This is measured by blood tests. Another purpose is to find out if nipocalimab can cause side effects, which are unexpected or unwanted reactions from taking a drug.

Participants will first be enrolled into the Double-Blind Placebo-Controlled Treatment Period which will last for about 24 weeks. Participants in this period of the study will receive either nipocalimab or a placebo through an infusion (IV). Participants with come for an office visit every 2 weeks to obtain the infusion, complete questionnaires, perform safety assessments and lab tests.

At the end of the Double-Blind treatment period, your study doctor will determine if you are eligible to enroll in the Open-Label Extension (OLE) period in which you would be able to receive nipocalimab infusions for approximately 2-5 years. There also may be a chance you did not participate in the Double-Blind Treatment Period of this study but can still enroll in the Open-Label Extension. Participants will complete the same activities in the OLE has the Double-Blind Placebo-Controlled Treatment Period.

The total amount of time the subject would be on the study is about 6 months for the Double-Blind Placebo-Controlled Treatment Period with an additional 2-5 years for the Open-Label Extension (OLE) period.