Note: During maturation thymocytes express on their surface a precursor T-cell receptor (TCR) called a pre-T-cell receptor, and both CD8 and CD4 proteins (double-positive cells). At the same time they slowly migrate from the outer cortex of the thymus to the inner medullary region. Only the \(\pu{2\%}\) of cells capable of distinguishing self-antigen survive to maturity in the medulla. Here they become single-positive cells, expressing exclusively CD4 or CD8 surface proteins, and are exported from the thymus in the bloodstream to become helper T cells (Th) and cytotoxic T cells, respectively.