MICA and osteosarcoma: In addition, human leukocyte antigen (HLA) class I (a ligand for inhibitory receptors on NK cells) is typically downregulated (3), while major histocompatibility complex class I chain-related protein A/B (MICA/B) and UL16-binding protein (ULBP) (ligands for activating receptors on NK cells) (60, 61) are overexpressed on osteosarcoma cells, allowing for easy activation of NK cells.