Postthymic proliferation of human naive CD4⁺ T cells. RTEs emigrate from the thymus and are incorporated into the pool of TREC^hi CD31^{+ thymic}naive CD4⁺ T cells. Some CD31^{+ thymic}naive CD4⁺ T cells undergo peripheral postthymic selection; long-lived TREC^lo CD31^{− central}naive CD4⁺ T cells are induced and maintain the size of the naive CD4⁺ T-cell pool. On cognate interaction with foreign antigens, both subsets can differentiate directly into memory/effector CD4⁺ T cells.