Thymocytes, precursors to T cells, develop in the thymus. They undergo several stages of selection and quality control to ensure maturation results in functional T cells. The first step is beta-selection in the cortex of the thymus which tests the structural properties of the newly formed T cell receptor (TCR) beta chain. The TCR structure must allow presentation on the cell surface with pre-TCR alpha. Thymocytes are then subjected to positive selection, where reactivity and selectively are dependent on functional interactions with the major histocompatibility complex (MHC). Final maturation of thymocytes occurs in the medulla, where thymocytes with high affinity for self-peptide or the MHC are eliminated from the population by apoptosis. Once thymocytes are released into the peripheral blood stream they are considered mature T cells.