The transcription factor Nur77, an orphan member of the nuclear hormone receptor superfamily, is highly expressed during T cell receptor-signaled apoptosis, suggesting a possible role for Nur77 in negative selection. We examined this by generating two sets of transgenic mice. In one set of mice, a dominantnegative Nur77 mutant is constitutively expressed and the other in which wild-type Nur77 protein is constitutively expressed in developing thymocytes. We report that inhibition of endogenous Nur77 by the dominantnegative mutant perturbed T cell development and inhibited antigen-induced negative selection in F5 T cell receptor transgenic mice. Constitutive expression of wild-type Nur77 protein induced apoptosis in developing thymocytes, resulting in a decreased number of thymocytes and mature T cells. Together, these data support a role for Nur77 in the downstream signaling events in antigen-induced negative selection.