CMV infection leaves a footprint on the phenotype of the NK cell compartment, leading to an increase in mature NK cells expressing the activating receptor NKG2C, which specifically recognize CMV infected cells, and expand after CMV reactivation.18–24 We have previously shown that CMV-specific CD4+ and CD8+ T cell subsets expand in CLL, whereas their anti-CMV activity is unaffected.25–27 The failure of other components of the immune system to control CMV may explain the expansion of CMV-specific T cells in CLL; for example reduced immunosurveillance by NK cells. The gene discussed is CD4; the disease is B-cell chronic lymphocytic leukemia.