Development of CD4⁺CD25⁺ cells during human fetal ontogeny. (A) Thymic and splenic T cells from fetuses of the indicated ages and from one 8-week-old newborn were analyzed by flow cytometry for CD4 and CD25 expression. Density plot windows shown are gated on CD3⁺ lymphocytes. (B) Successive gates used to determine the proportion of CD25^hi cells among CD3^hiCD4⁺CD8^-SP cells reported in Table 1 are shown.