KLRK1 and neoplasm: There are a variety of activated receptors on the surface of NK cells, such as natural killer group 2 member D (NKG2D) and natural cytotoxicity receptors (NCRs; NKp46, NKp44 and NKp30) (221), etc. Among them, NKG2D is one of the most studied receptors and is able to recognize ligands on the surface of tumour cells, such as major histocompatibility complex class I polypeptide-related sequence A and B (MICA/B) and UL16-binding protein (ULBP).