Over the past decade, a number of important advances have been made in the molecular characterization and the treatment of the primary immunodeficiency disorders. These advances include identification of the abnormal genes responsible for such syndromes as X-linked severe combined immune deficiency, several forms of autosomal severe combined immune deficiency, X-linked and autosomal agammaglobulinemia, Wiskott-Aldrich syndrome, and other primary immunodeficiency disorders. In the past year, the biologic functions of the abnormal gene products responsible for these syndromes have been better defined, and new molecular defects that lead to primary immunodeficiency disorders have also been reported. This better understanding of the molecular basis of the primary immunodeficiency disorders has led to improvement of established therapies, such as gene product replacement and stem cell transplants, and to new treatment strategies, such as gene therapy.