In patients with X-linked hyper-IgM syndrome, mutations in the CD40L gene result in disrupted CD40/CD40L signaling, leading to abortive germinal center reactions, significant depletion, and phenotypical abnormalities of follicular dendritic cells, thereby impairing the functional development of B-cell follicles. The gene discussed is CD40LG; the disease is X-linked hyper-IgM syndrome.