Because (a) Runx3 appeared necessary for Eomes induction (Fig. 3 A), (b) the kinetics of Eomes expression paralleled those of perforin expression (Fig. 2), and (c) overexpression of Eo-VP16 in either WT or T-bet–deficient T cells led to an increase in both perforin and IFN-γ expression (Fig. 2, B and D), we asked whether CTL differentiation was also potentially regulated by a feed-forward loop involving these same two classes of Runx and T-box transcription factors.