KLRK1 and neoplasm: Thus, in addition to the up-regulation of PD-L1, the down-regulation of NKG2D activating ligands such as UL16 binding protein 1 (ULBP1), ULBP2, ULBP3, major histocompatibility complex class I chain-related molecules A and B (MICA, and MICB), is another way for tumor cells to escape from NK cell-mediated cytotoxic action [6].