This study was intended to elucidate the functional maturation process of human regulatory T (Treg) cells through the pathogenic examination of congenital deficiency of Treg cells (IPEX). We developed the simple and easy method by flow cytometry for detection of IPEX patients, resulting in identification of some new IPEX cases in Japan. Based on the analyses of IPEX patients, the IPEX case undergoing bone marrow transplantation, carriers, newborns and healthy adults, it could partially be clarified that FOXP3, the product of the gene responsible for IPEX, plays a crucial role in development and differentiation of Treg cells in humans.