The mechanism that plays an essential role in immunosuppression and regulation is the presence of naturally arising CD4+CD25+ T lymphocytes (Treg). Quantitative or qualitative dysfunc- tions in these cells may lead to autoimmune diseases. The Foxp3, a forkhead/winged helix (FKH) trans- cription factor, is the key regulatory gene for the development and function of regulatory T lymphocytes. The Foxp3 expression is limited to CD4+CD25+ T cells at both mRNA and protein levels. The Foxp3 interacts with nuclear factor of activated T cells (NFAT) and repress cytokine gene expression. Iduction of the Foxp3 expression in human CD4+CD25- T cells can convert these cells to a regulatory T cell phenotype. This finding may provide new insight into the biology of regulatory T cells and could become a useful therapeutic tool in the treatment of autoimmunity.