effector cytokine and cytolytic genes.
Runx proteins, a family of three DNA-binding transcription factors, control thymocyte differentiation and the CD4/CD8 lineage decision (10–13). Runx3 and perforin mRNA are expressed by double-positive (DP) thymocytes and CD8+ single-positive (SP) thymocytes but not in CD4+ SP cells (14). Although Runx3 is not expressed in naive CD4+ T cells, its expression is up-regulated during Th1 cell differentiation, and Runx3 influences T