Acute meningitis or meningoencephalitis was diagnosed in 4 cases (one with intrathecal synthesis of IgG, IgA and IgM detected in ventricular CSF only, one with an intrathecal IgM synthesis both in lumbar and ventricular CSF, 2 without detectable intrathecal immunoglobulin synthesis). The gene discussed is CD40LG; the disease is meningoencephalitis.