Natural killer cell expansion during primary EBV infection has first been reported in a study by Tomkinson et al. (42), in which peripheral NK cells (identified as CD16+ lymphocytes) were described to be significantly increased in both frequency (1.5-fold) and absolute number (4-fold) in – by these measures – a similar manner to CD8+ T cells in a cohort of IM patients. The gene discussed is FCGR3A; the disease is Epstein-Barr virus infection.