Thymic selection of the developing T cell repertoire is thought to occur at the CD4+CD8+ stage of differentiation and to be determined by the specificity of the T cell receptors (TCRs) that CD4+CD8+ thymocytes express. However, TCR signals can inhibit the differentiation of precursor thymocytes into CD4+CD8+ cells, which suggests that selection might occur earlier than thought. Indeed, in a negatively selecting male thymus, CD4-CD8lo precursor thymocytes that express a transgenic TCR to male antigen are developmentally arrested as a consequence of antigen encounter and fail to become CD4+CD8+. Thus, negative selection can occur before the CD4+CD8+ stage of differentiation.