Several genetic defects in class switch recombination, which lead to a hyper-IgM syndrome, have been described recently in humans. In addition to the well known role of CD40-ligand-CD40 interaction, these pathologies demonstrate definitively the requirement of CD40-mediated nuclear factor kappa B activation and the essential role of a recently described molecule, the activation-induced cytidine deaminase in an efficient humoral response, which includes class switch recombination and the production of high-affinity antibodies.