This study is researching an experimental drug called REGN7999 (called study drug). The study is focused on patients with non-transfusion dependent beta-thalassemia. The aim of the study is to see how safe and effective the study drug is. The study is looking at several other research questions, including: * Whether the study drug lowers extra iron levels in the body * What side effects may happen from taking the study drug * How much study drug is in the blood at different times * Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)