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The concept of tumor immuno-surveillance suggests that the natural immune system can detect and eliminate a majority of malignant cancer cells and provide a first line of defense against tumor development.Natural killer (NK) cell immunoglobulin-like receptors (KIRs) are members of the immunoglobulin superfamily of receptors expressed on NK cells.NK cells recognize major histocompatibility complex (MHC) class 1 molecules in normal cells via KIRs, which deliver signals that suppress or inhibit NK cells from killing normal "self" cells.NK ceils can kill those cancer cells that have lost or express insufficient amounts of MHC class 1.The expression of KIR receptors and its activation or inhibition of NK functions is unique to NK cells.However, we have unexpectedly detected that NK immunoglobulin-like inhibitor receptors (KIRs) become expressed on the surface of human lung cancer cells after orthotopic implantation in the lungs of athymic nude rats.Levels of KIR expression increase progressively with distance of metastases from primary tumor and correlate strongly with metastatic behavior.In contrast, KIR is very low or not detectable in the parental lung cancer cell line in vitro.