TNFRSF9 and neoplasm: CD137 is expressed by multiple myeloid cells including activated effector CD8+ and CD4+ T cells, natural killer (NK) cells, NK/T cells, dendritic cells (DCs), macrophages, neutrophils, eosinophils [4], and according to recent data, also by regulatory T cells (Tregs), activated B cells, mast cells and endothelial cells in tumor capillaries [4]–[7].