This study is observational and will use the results of EBV PCR and EBV serology allowing our objectives to be achieved with considerable cost reduction. Time zero is when the patient receives his graft. Some tests are done before time zero (pre-transplant array of test including EBV serology). After time zero, in all sites, there is a follow-up treatment protocol including EBV PCR testing. Pre-transplant sera from recipients and donors are tested by standard methods for mmunoglobulin G (IgG) antibodies to the EBV capsid antigen (VCA), IgG antibodies to EBV early antigens (EA) and anticomplement antibodies to the EBV nuclear antigens (EBNA). EBV DNA testing is performed by quantitative polymerase chain reaction (PCR). Each centre has an established threshold value for determining viral load. In order to properly interpret the patients' viral load data from each participating center, all will be required to fill out a site report form (SRF) to describe their testing method for EBV PCR and EBV serology.