In immunocompetent recipient mice, naïve CD4+TCR-HA+ T cell transfer (Figure 1(a)) fails to induce overt diabetes (Figure 1(c)), perhaps due to extrathymic induction of a Foxp3+ Treg cell phenotype in a significant proportion of initially Foxp3− T cells, upon recognition of the cognate antigen on antigen-presenting cells residing in peripheral lymphoid tissues (Figure 1(b)). The gene discussed is CD4; the disease is diabetes mellitus.