CD4⁺ regulatory T cells suppress self-reactive lymphocytes and control immunological self-tolerance. Their role is thought to be important in the etiology autoimmune diseases, but the molecular mechanisms and genes involved in the development of these regulatory T (TR) cells have been unclear. In the January 9 Sciencexpress, Shohei Hori and colleagues at The Institute for Physical and Chemical Research (RIKEN), Yokohama, Japan, show that the transcription factor gene FOXP3 controls TR cell development (Sciencexpress, 10.1126/science.1079490, January 9, 2003).