KLRK1 and cytomegalovirus infection: Similar to others, we observe a reduced frequency of NK cells expressing the activating cytotoxicity receptor NKG2D in CLL patients (Fig. 2E).13–17 Although CMV infection seems to have a role in downregulation of NKG2D in HC and CLL, the frequency of NKG2D+ NK cells is also reduced in CLL patients not infected with CMV, suggesting a direct tumor effect on NKG2D expression (Fig. 2F).