Notch proteins influence cell-fate decisions in many developing systems. Several gain-of-function studies have suggested a critical role for Notch1 signaling in CD4-CD8 lineage commitment, matura-tion and survival in the thymus. However, we show here that tissue-specific inactivation of the gene encoding Notch1 in immature (CD25+ CD44−) T cell precursors does not affect subsequent thymocyte development. Neither steady-state numbers nor the rate of production of CD4+ and CD8+ mature thymocytes is perturbed in the absence of Notch1. In addition, Notch1-deficient thymocytes are normally sensitive to spontaneous or glucocorticoid-induced apoptosis. In contrast to earlier re-ports, these data formally exclude an essential role for Notch1 in CD4-CD8 lineage commitment, maturation or survival.