In SCID patients with defective V(D)J somatic recombination, who cannot generate antigen-specific T and B cells, NKG2C+ NK cell expansion might provide a marker for the detection of an HCMV-induced immune response in some of these patients, although we cannot exclude the role of maternal T cells in the generation of NKG2C+ NK cells in patient 1. This evidence concerns the gene KLRC2 and severe combined immunodeficiency.