**5. Concluding remarks(**

NK cells not only are important players of innate effecter responses, but also participate in the initiation and development of antigen-specific responses through secretion of immunoregulatory cytokines (such as IFN-, tumor necrosis factor- (TNF-) and granulocyte–macrophage colony-stimulating factor (GM–CSF)) and through cell-to-cell contact (Biron et al., 1999). Regulation of the expression of NK-cell receptors and their ligands refers to the control of the amount, the timing of appearance, and the functional products of these genes, leading to the flexibility of NK cells or other immune cells to adapt to a variable environment, external signals, and damages to target cells (Wu et al., 2005). In particular, the modification of NK-cell receptors and their ligands is a potential therapeutic implication for cancer treatment. Although the regulation mechanism of expression of NKcell receptors and related ligands has acquired important progress at the transcription level, the regulatory mechanisms of a number of crucial receptors and ligands, for example signaling lymphocytic activating molecule (SLAM)-related receptors (CD150, 2B4, Ly-9,

CD84, and NTB-A) (Sidorenko and Clark, 2003; Engel et al., 2003; Morra et al., 2001), are not well understood. More research is needed into other regulated stages of gene expression, such as posttranscriptional modification, RNA transport, translation, and mRNA degradation. Investigation of the regulatory mechanisms of NK-cell receptors and their ligands might aid in the design of therapy against cancer, infection, inflammation, or autoimmune diseases.
