As expansion of NK cells following viral infections has been largely described for activating NK cell receptors, including KIR3DS1 and NKG2C in humans and Ly49H in mice (14, 15, 30, 31), the capacity of NK cells to expand might be attributed to DAP-12, the adaptor molecule mediating signal transduction of activating receptors (30). This evidence concerns the gene KLRD1 and viral infectious disease.