Analysis of the INK4A/ARF locus in human T-ALL patients revealed frequent deletions in exon 2, the exon common to both p16 INK4A and p14 ARF . Other studies have described selective deletion of exon 1b of p14 ARF or methylation of the p16 INK4A promoter. Therefore, it is unclear from these studies whether loss of p16 INK4A and/or p14 ARF contributes to the development of T-ALL. To elucidate the relative contribution of the ink4a/arf locus to T-cell leukemogenesis, we mated our tal1 transgenic mice to ink4a/arfĂ/Ă, p16 ink4a Ă/Ă, and p19 arf Ă/Ă mice and generated tal1/ink4a/arf ĂŸ /Ă, tal1/p16 ink4a ĂŸ /Ă, and tal1/p19 arf ĂŸ /Ă mice. Each of these mice developed T-cell leukemia rapidly, indicating that loss of either p16 ink4a or p19 arf cooperates with Tal1 to induce leukemia in mice. Preleukemic studies reveal that Tal1 expression stimulates entry into the cell cycle and thymocyte apoptosis in vivo. Interestingly, mice expressing a DNA-binding mutant of Tal1 do not exhibit increases in S phase cells. The S phase induction is accompanied by an increase in thymocyte apoptosis in tal1 transgenic mice. Whereas apoptosis is reduced to wild-type levels in tal1/ink4a/ arfĂ/Ă mice, S phase induction remains unaffected. Thus, Tal1 stimulates cell cycle entry independent of the ink4a/arf locus, but its ability to induce apoptosis is Ink4a/Arfdependent.