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Frequent hallmarks of T-cell acute lymphoblastic leukemia (T-ALL) include aberrant NOTCH signaling and deletion of the CDKN2A locus, which contains 2 closely linked tumor suppressor genes (INK4A and ARF). When bone marrow cells or thymocytes transduced with a vector encoding the constitutively activated intracellular domain of Notch1 (ICN1) are expanded ex vivo under conditions that support T-cell development, cultured progenitors rapidly induce CD4+/CD8+ T-ALLs after infusion into healthy syngeneic mice. Under these conditions, enforced ICN1 expression also drives formation of T-ALLs in unconditioned CD-1 nude mice, bypassing any requirements for thymic maturation. Retention of Arf had relatively modest activity in suppressing the formation of T-ALLs arising from bone marrow-derived ICN1+ progenitors in which the locus is epigenetically silenced, and all resulting Arf (+/+) tumors failed to express the p19(Arf) protein. In striking contrast, retention of Arf in thymocyte-derived ICN1+ donor cells significantly delayed disease onset and suppressed the penetrance of T-ALL. Use of cultured thymocyte-derived donor cells expressing a functionally null Arf-GFP knock-in allele confirmed that ICN1 signaling can induce Arf expression in vivo. Arf activation by ICN1 in T cells thereby provides stage-specific tumor suppression but also a strong selective pressure for deletion of the locus in T-ALL.