H yper-IgM syndrome (HIGM) is a primary immunodeficiency characterized by normal to elevated serum levels of IgM and low levels or the absence of IgG, IgA, and IgE. A new study AID expression in nonlymphoid cells (see related article on pages 136–142) characterizes HIGM type 4, a previously undocumented defect in antibody gene diversification caused by a selective block in class-switch recombination, providing significant insight towards understanding HIGM immunodeficiencies.