Following the discovery of the molecular basis of X-linked hyper-IgM syndrome, descriptions of male and female patients with normal CD40L expression, increased susceptibility to bacterial but not opportunistic infections, and, in some families, an autosomal recessive inheritance pattern appeared. In 2000, Revy et al. published the results of a study of such a group of patients with hyper-IgM syndrome, which revealed a mutation in the gene encoding activation-inducible cytidine deaminase (AICDA).