The Nur77/Nurr1 family of DNA binding proteins has been reported to be required for the signal transduction of CD3/T cell receptor (TCR) - mediated apoptosis in T cell hybridomas. To determine the role of this family of DNA-binding proteins in thymic clonal deletion, transgenic (Tg) mice bearing a dominant negative mutation were produced. The transgene consisted of a truncated Nur77 (ΔNur77) gene encoding the DNA-binding domain of Nur77 ligated to a TCR-β enhancer resulting in early expression in thymocytes. Apoptosis of CD4+CD8+ thymocytes mediated by CD3/TCR signaling was greatly inhibited in the ΔNur77 Tg mice, compared with non-Tg littermates, after treatment with anti-CD3 or anti-TCR antibody in vivo and in vitro. Clonal deletion of self-reactive T cells was investigated in ΔNur77-Db/HY TCR-α/β double Tg mice. There was a five-fold increase in the total number of thymocytes expressing self-reactive Db/HY TCR-α/β in the ΔNur77-TCR-α/β double Tg male mice. Deficient clonal deletion of self-reactive thymocytes was demonstrated by a 10-fold increase in the CD4+CD8+ thymocytes that expressed Tg TCR-α/β. There was an eight-fold increase in CD8+, Db/HY TCR-α/β T cells in the lymph nodes (LN) of ΔNur77-Db/HY TCR-α/β double Tg compared with Db/HY TCR-α/β Tg male mice. In spite of defective clonal deletion, the T cells expressing the Tg TCR were functionally anergic. In vivo analysis revealed increased activation and apoptosis of T cells associated with increased expression of Fas and Fas ligand in LN of ΔNur77-Db/HY TCR-α/β double Tg male mice. These results indicate that inhibition of Nur77/Nurr1 DNA binding in T cells leads to inefficient thymic clonal deletion, but T cell tolerance is maintained by Fas-dependent clonal deletion in LN and spleen.