Clinical data that support a decisive role for CMV in driving the expansion of memory NK cells in vivo are provided by studies that reported a strong increase of NKG2C+ NK cells in solid organ transplanted patients, during acute CMV infection [226], or in bone marrow transplant recipients, following acute HCMV infection or reactivation [227,228,229,230]. This evidence concerns the gene KLRC2 and cytomegalovirus infection.