The rare syndrome known as IPEX (OMIM: 304930) is characterized by immune-dysfunction, polyendocrinopathy, enteropathy, and X-linked inheritance. The gene responsible for IPEX maps to Xp11. 23-q13. 3, a region of the X chromosome that also harbors the Wiskott-Aldrich syndrome gene (WASP). IPEX syndrome results from mutations of a unique DNA binding protein gene, FOXP3. Mutations invariably impair the seemingly essential forkhead domain of the protein, which is uniquely located in the carboxyl terminus, affecting protein function. In this review, we describe the identification of IPEX as a unique X-linked syndrome, the clinical features of IPEX, mutations of the immune-specific FOXP3 DNA binding protein, and bone marrow transplantation as a potential cure for the syndrome, which is usually lethal within the first year of life in affected males.