Laboratory tests showed that the levels of serum immunoglobulins (IgG, IgA, IgM, and IgE) and antibodies related to autoimmune diseases were normal, but the level of C4 was slightly high (0.458 g/L, normal range 0.16–0.38 g/L) and positivity to anti-EBV-CAIgG suggested previous Epstein-Barr virus (EBV) infection. The gene discussed is C4A; the disease is Epstein-Barr virus infection.