Healthy volunteers
A total of 410 healthy children (227 males, 183 females) aged 0–18 years were recruited. All were healthy volunteers from Chongqing, China. We defined “healthy” as follows: normal nutritional status, no evidence of infectious disease, no medication, no contact with persons with infectious disease, no immunizations for at least the last 4 weeks, no blood transfusions, no history of atopic disorders, and no evidence of hematological or immunological disorders. All participants underwent regular blood tests, liver and renal function tests, serological tests for hepatitis B virus, and real-time PCR for EBV and cytomegalovirus. Blood samples were obtained from September 2014 to June 2015. DNA was extracted from peripheral blood.