KLRK1 and Ewing sarcoma: Transformed cells, including Ewing's sarcoma,24 typically express membrane bound NKG2D ligands facilitating this costimulation and sensitizing them to cytotoxic killing; however, some tumors including neuroblastoma have been demonstrated to shed soluble NKG2D ligands which can block NKG2D cross-linking and therefore abrogate this essential second signal.9