Immunodysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome is a rare disorder caused by mutations in theFOXP3gene that result in the defective development ofCD4+CD25+regulatory T cells which constitute an important T cell subset involved in immune homeostasis and protection against autoimmunity. Their deficiency is the hallmark of IPEX and leads to severe autoimmune phenomena including autoimmune enteropathy, dermatitis, thyroiditis, and type 1 diabetes, frequently resulting in death within the first 2 years of life. Apart from its clinical implications, IPEX illustrates the importance of immunoregulatory cells such asCD4+CD25+regulatory T cells.