['Naturally occurring thymus derived regulatory T cells (Tregs) are central in the maintenance of self-tolerance. The transcription factor FOXP3 is crucial for the suppressive activity of Tregs and is considered the most specific marker for this population. However, human non regulatory T cells upregulate FOXP3 transiently upon activation which calls for other means to identify the Treg population. Since epigenetic mechanisms are involved in the establishment of stable gene expression patterns during cell differentiation, we hypothesized that the methylation profile of the FOXP3 promoter would allow the distinction of truly committed Tregs.', 'Human CD4(+)CD25(hi) Tregs displayed a demethylated FOXP3 promoter (1.4%+/-0.95% SEM methylated) in contrast to CD4(+)CD25(lo) T cells which were partially methylated (27.9%+/-7.1%). Furthermore, stimulated CD4(+)CD25(lo) T cells transiently expressed FOXP3 but remained partially methylated, suggesting promoter methylation as a mechanism for regulation of stable FOXP3 expression and Treg commitment. In addition, transient FOXP3 expressing cells exhibited suppressive abilities that correlate to the methylation status of the FOXP3 promoter. As an alternative to bisulphite sequencing, we present a restriction enzyme based screening method for the identification of committed Tregs and apply this method to evaluate the effect of various culturing conditions. We show that a partial demethylation occurs in long-term cultures after activation, whereas the addition of TGF-beta and/or IL-10 does not induce any additional change in methylation level.'] Does fOXP3 promoter demethylation reveal the committed Treg population in humans?