The function of KLRK1 (killer cell lectin like receptor K1, Ensembl gene identifier ENSG00000213809) is as follows. Functions as an activating and costimulatory receptor involved in immunosurveillance upon binding to various cellular stress- inducible ligands displayed at the surface of autologous tumor cells and virus-infected cells. Provides both stimulatory and costimulatory innate immune responses on activated killer (NK) cells, leading to cytotoxic activity. Acts as a costimulatory receptor for T-cell receptor (TCR) in CD8(+) T-cell-mediated adaptive immune responses by amplifying T-cell activation. Stimulates perforin-mediated elimination of ligand-expressing tumor cells. Signaling involves calcium influx, culminating in the expression of TNF. Participates in NK cell-mediated bone marrow graft rejection. May play a regulatory role in differentiation and survival of NK cells. Binds to ligands belonging to various subfamilies of MHC class I-related glycoproteins including MICA, MICB, RAET1E, RAET1G, RAET1L/ULBP6, ULBP1, ULBP2, ULBP3 (ULBP2>ULBP1>ULBP3) and ULBP4. {ECO:0000269|PubMed:10426994, ECO:0000269|PubMed:11224526, ECO:0000269|PubMed:11777960, ECO:0000269|PubMed:15240696, ECO:0000269|PubMed:19658097, ECO:0000269|PubMed:21898152, ECO:0000269|PubMed:23298206, ECO:0000269|PubMed:28559451}.