U266/STAT5 Reporter (GFP) cells are derived from the human myeloma U266 cell line by stably integration of a STAT5 GFP 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/STAT5 Reporter (GFP) cells stably express GFP under the control of the STAT5 response elements, can be used for monitoring the activity of STAT5 signaling pathway.