The 3G3 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 expressed at constitutively high levels in Treg cells, which are further identified as being CD4+ CD25+. In resting conventional T cells (CD4+ CD25-) Foxp3 expression is restricted, and upon TCR activation is expressed only transiently and in a small proportion of cells. However, the growth factor TGF-beta has been shown to induce expression of Foxp3 in naïve T cells, driving their development into Foxp3+ Tregs, which are called “induced” or “adaptive” Tregs. These cells are phenotypically similar to so-called “natural” Tregs (CD4+ CD25high Foxp3+) which originate in the thymus and comprise the majority of Treg cells. Tregs are critical for maintaining peripheral tolerance and are implicated in the development of autoimmunity.