Increased expression of NKG2C is thought to correlate with human cytomegalovirus (HCMV) infection,70, 71, 72 and while total NK cells (CD3−CD19− cells) increased in early life (2 months to 4 years), the proportion of NKG2C+ NK cells did not significantly increase after birth (0–1 month; Supplementary figure 5a, b). Here, KLRC2 is linked to cytomegalovirus infection.