KLRK1 and osteosarcoma: The downregulation of NK cell activating receptors in the periphery may result in diminished anti-tumor responses and can be reversed by ex vivo activation of NK cells expressing DNAM-1 and NKG2D, that have been shown to efficiently target and kill Ewing sarcoma (EWS), rhabdomyosarcoma (RMS) and osteosarcoma cell lines (derived from patient tumor samples) in vitro (27, 63, 64).