CD40LG and Epstein-Barr virus infection: For the nine patients with a negative EBV PCR test from a CSF sample, the diagnosis of EBV encephalitis was based on one of the following tests: (i) the presence of an anti-EBV IgM test highlighting primary EBV infection (six patients); (ii) a positive EBV qPCR test from a brain biopsy sample 98 days after stem-cell transplantation (one patient); (iii) a positive EBV PCR test on a sample from a second lumbar puncture procedure, performed after 12 or 16 days (two patients).