The purpose of this study is to find out if the study drug, MAS825, is effective in preventing flares and safe for children with NLRC4-Gain of Function (GOF). Researchers will also be testing how the body processes a drug by measuring how quickly the drug is absorbed by the body, how quickly it is broken down by the body, and how long it remains in the body (pharmacokinetics). Subjects will be asked to come into the Rheumatology clinic for blood and stool testing, physical exam, an electrocardiogram (ECG), questionnaire completion, and administration of study drug. Participation will last for up to 4 years.