U266/STAT3 Reporter (Luc) cells are derived from the human myeloma U266 cell line by stably integration of a STAT3 firefly luciferase reporter construct. The U266 cells are an important tool in studying the pathogenesis, progression, and treatment of multiple myeloma, as well as for testing chemotherapies and immunotherapies, including monoclonal antibodies, immune checkpoint inhibitors, and CAR-T cell therapies. U266/STAT3 Reporter (Luc) cells stably express firefly luciferase under the control of the STAT3 response elements, can be used for monitoring the activity of STAT3 signaling pathway.