tokens: Our results separate these factors into four regulatory classes : ( i ) constitutive factors , such as Oct - 1 and probably Sp1 , that are expressed in thymocytes at all stages ; ( ii ) inducible factors , such as NF - kappa B and complexes binding to the region of a CD28 response element , that can be activated in all thymocytes , including those cells ( CD4 + CD8 + TcRlow ) that can undergo selection ; ( iii ) inducible factors , such as NF - AT and AP - 1 , that can be activated in mature ( CD4 + CD8 - TcRhigh ) and immature ( CD4 - CD8 - TcR - ) thymocytes alike but not in the transitional stages when the cells ( CD4 + CD8 + TcRlow ) are subject to selection ; and ( iv ) a factor containing CREB , which can be activated in thymocytes of all developmental stages by culture but does not require specific induction .