The ternary complex factors (TCFs; SAP-1, Elk-1, and Net) are serum response factor cofactors that share many functional properties and are coexpressed in many tissues. SAP-1, the predominant thymus TCF, is required for thymocyte positive selection. In this study, we assessed whether the different TCFs are functionally equivalent. Elk-1 deletion, but not the hypomorphic Net δ mutation, exacerbated the SAP-1 positive selection phenotype, but triply deficient thymocytes were no more defective than SAP-1−/− Elk-1−/− cells. Inactivation of the other TCFs did not affect SAP-1–independent processes, including β-selection, regulatory T cell selection, and negative selection, although reduced marginal zone B cells were observed in SAP-1−/− Elk-1−/− animals. Ectopic expression of Elk-1, but not Net, rescued positive selection of SAP-1−/− thymocytes; thus, SAP-1 and Elk-1 are functionally equivalent in this system, and the SAP-1 null selection phenotype reflects only its high expression in the thymus. Array analysis of TCR-stimulated double-positive cells identified SAP-1–dependent inducible genes whose transcription was further impaired in SAP-1−/− Elk-1−/− cells; thus, these genes, which include Egr-1 and Egr-2, represent candidate mediators of positive selection. Chromatin immunoprecipitation revealed subtly different promoter targeting between the different TCFs. Ectopic expression of Egr-1 restored positive selection in SAP-1 null thymocytes, establishing it (and possibly other Egr family members) as the major effector for ERK–SAP-1 signaling in thymocyte positive selection.