PVR and neoplasm: NK activating receptors recognize stress molecules (e.g., MIC-A/B and UL16-binding protein 1-4, ULBP1-4, recognized by NKG2D and CD112 and CD155 recognized by DNAM-1) upregulated on many tumor cells and trigger tumor targeting,50, 51, 52 while NK inhibitory receptors recognize matched “self” major histocompatibility complex (MHC) molecules and suppress tumor killing.53