The MF23 antibody reacts with mouse Foxp3, a 50-55 kDa transcription factor which is a central regulator of T cell activity and is critical for the development and function of regulatory T cells (Tregs). Foxp3 is a member of the forkhead box or winged helix family of transcription factors and is expressed at constitutively high levels in Treg cells. Forced expression of Foxp3 in conventional T cells results in the upregulation of Treg associated molecules such as CD25, CTLA-4, and GITR and is sufficient to impart suppressive functional activity to these cells.