Antigen-binding domains that are capable of binding to CD3 and CD137 but do not bind to CD3 and CD137 at the same time and methods of using the same are provided. Methods to obtain antigen binding domains which bind to two or more different antigens more efficiently are also provided.提供可結合至CD3及CD137但不同時結合至CD3及CD137的抗原結合域及其使用方法。亦提供更有效率地獲得結合二種或更多種抗原的抗原結合域的方法。