Naïve BDC2.5+ T cells, FACS purified (Figure 2(a)) from peripheral lymphoid tissues of immunocompetent NOD.BDC2.5 mice with Foxp3-dependent GFP expression (see Section 4.3) and adoptively transferred into either TCR-β−/− [28] or Rag1−/− (Figure 2(b)) NOD mice, undergo lymphopenia-driven proliferation, resulting in the acquisition of a Foxp3+ Treg cell phenotype in a significant proportion of initially Foxp3− T cells [29–31]. Here, FOXP3 is linked to lymphopenia.