The lack of tools for direct identification of regulatory T cells (T_reg cells) at the single‐cell level has been one of the major hurdles for the study of T_reg cells and their involvement in human disease. The identification of the transcription factor Foxp3 as a molecular correlate for T_reg function offered the opportunity to directly identify T_reg cells, but until recently adequate reagents were not available. The tools promising the solution for this problem have emerged through the development of transgenic mice in which Foxp3 expression drives the production of fluorescent proteins, as well as the development of mAb that are able to identify Foxp3⁺ cells by histology or flow cytometry. With these new tools, the mAb in particular, it will become possible for the first time to directly probe the participation of Foxp3⁺ T_reg cells in human pathology in a quantitative fashion.

See accompanying article: http://dx.doi.org/10.1002/eji.200526189