KLRD1 and cytomegalovirus infection: NK cell differentiation spans from naive CD56bright cells to highly differentiated CD56dim adaptive NK cells induced by cytomegalovirus (CMV) infection.15 16 Adaptive NK cells are characterized by the expression of the activating receptor CD94/NKG2C (NKG2C), whose cognate ligand HLA-E also binds to the inhibitory receptor CD94/NKG2A (NKG2A).